Electro Decomposition of Ammonia Into Hydrogen

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Electro Decomposition of Ammonia

into Hydrogen for Fuel Cell Use

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Gerardine G. Botte, and Carl A. Feickert January 2012

Approved for public release; distribution is unlimited.





ERDC/CERL TR-12-1

January 2012

Electro Decomposition of Ammonia

into Hydrogen for Fuel Cell Use

Gerardine G. BotteCenter for Electrochemical Engineering Research

Ohio University

165 Stocker Center

Athens, OH 45701

Carl A. FeickertConstruction Engineering Research Laboratory (CERL)

U.S. Army Engineer Research and Development Center

2902 Newmark Dr.

Champaign, IL 61822-1076

Final Report

Approved for public release; distribution is unlimited.

Prepared for Headquarters, U.S. Army Corps of Engineers

Washington, DC 20314-1000

Under Work Unit 0J4070



ERDC/CERL TR-12-1 ii

Abstract

This work was undertaken to create an efficient process for electrolyzing

ammonia, by clarifying the electrolytic decomposition pathways of

ammonia and urea. This project demonstrated the feasibility of using

ammonia and urea electrolysis technologies to produce hydrogen as a

potential fuel source for the fuel Proton Exchange Membrane (PEM) fuel

cell back up power for training facilities and soldier camps, under the

“Silent Camp” initiative. This was achieved with scaling of bench scale

electrolyzer to a 50 W electrolyzer system known as the “GreenBox.” The

construction of the 50 W GreenBox depended on the development of the

catalyst and fundamental understanding of the reaction mechanisms forammonia and urea electrolysis. Significant progress in catalyst

development was achieved by using chemical and electrochemical

preparation techniques, and using the various state-of-the-art analytical

methods funded through this project. A new synthesized material–nickel

hydroxide nanosheets–has shown potential to be catalyst for urea

electrolysis and catalyst support for ammonia electrolysis. The energy

consumption for the ammonia electrolysis using the nickel based

nanostructured electrodes is anticipated to be lower than 8.6 Wh per gram

of hydrogen gas produced. The low energy consumption will provide a

significant advantage when the GreenBox is combined with fuel cells.



ERDC/CERL TR-12-1 iii

Table of Contents

Abstract ................................................................................................................................................... ii

List of Figures and Tables ...................................................................................................................... v

Preface .................................................................................................................................................. viii

1 Introduction ..................................................................................................................................... 1

1.1 Background ....................................................................................................................... 1

1.2 Objectives .......................................................................................................................... 2

1.3 Approach ............................................................................................................................ 2

1.4 Mode of technology transfer ............................................................................................. 3

2 Efficiency of Ammonia and Urea Electrolysis ............................................................................. 4

2.1 Ammonia electrolysis ........................................................................................................ 4

2.2 Recommendations regarding ammonia electrolysis ....................................................... 6

2.3 Urea electrolysis ................................................................................................................ 7

2.4 Recommendations for Phase II (urea electrolysis) ........... ........... .......... ........... ........... .... 8

3 Test of 50 W GreenBox ................................................................................................................ 10

3.1 Cell design ....................................................................................................................... 10

3.2 Single cell testing ............................................................................................................ 12

3.3 50 W GreenBox testing ................................................................................................... 15

3.4 Recommendations for Phase II (GreenBox) ................................................................... 20

4 The Reaction Mechanisms .......................................................................................................... 21

4.1 Electro-oxidation of ammonia ......................................................................................... 21

4.2 Recommendations for Phase II ...................................................................................... 26

4.3 Electro-oxidation of urea ................................................................................................. 26

4.4 Experimental research .................................................................................................... 29

4.5 Recommendations for Phase II (reaction mechanisms) ............................................... 31

5 Catalyst Development .................................................................................................................. 32

5.1 Ni-Rh catalyst .................................................................................................................. 32

5.2 Large size electrode ........................................................................................................ 32

5.3 Small sized electrode ...................................................................................................... 35

5.4 Ni-Co hydroxide catalyst .................................................................................................. 36

5.5 Ni(OH)2 nanosheets ........................................................................................................ 41

5.6 Recommendations for Phase II (catalyst development) ............................................... 43

6 Application of Ammonia and Urea Electrolysis in Wastewater Remediation ....................... 44

7 Conclusions and Recommendations ......................................................................................... 47

7.1 Conclusions ..................................................................................................................... 47

7.1.1 Improvement on the fundamental knowledge of the mechanisms of reactions .................. 47



ERDC/CERL TR-12-1 iv

7.1.2 Catalyst development .............................................................................................................. 48

7.1.3 Construction and scale up of a bench scale electrolyzer ...................................................... 49

7.2 Recommendations for Phase II or future work .............................................................. 50

Acronyms and Abbreviations .............................................................................................................. 51

References and Publications ............................................................................................................. 52

Report Documentation Page (SF 298) .............................................................................................. 56



ERDC/CERL TR-12-1 v

List of Figures and Tables

Figures

1 Experimental set up of the 9-cell stack of the ammonia electrolytic cell (AEC).

During ammonia electrolysis, the collection chambers at the cathode and anode

side accumulated the hydrogen and nitrogen gases, respectively ........................................ 7

2 Exploded view of the first large scale single cell designed to perform urea

electrolysis. The cell comprises of end plate (A), anode electrode (B), gasket

sheet (C), membrane (D), and cathode electrode (E) ........................................................... 11

3 Diagram of the new electrode for urea electrolysis. This electrode (Ni mesh #40)

is the base material for both anode and cathode electrodes of the GreenBox .................. 11

4 Sectional exploded view of the single cell assembled based on the new electrode

design. The anode electrode (A) and the cathode electrode (C) are separated bya membrane (B). The electrodes are covered to the external atmosphere by an

end plate (D) .............................................................................................................................. 12

5 Flow diagram of the single cell testing set up. The cathode electrolyte is

circulated through the cathode side of the cell whereas the anode electrolyte is

passed through the anode side only once (single pass) to evaluate the urea

conversion percentage ............................................................................................................. 12

6 Optimization plot for hydrogen production in a single pass testing of the single

urea electrolytic cell. Optimum values for obtaining maximum hydrogen

production are anode flow rate 7.8 mL/min, cathode flow rate 120 mL/min, and

cell voltage 1.6 V ....................................................................................................................... 15

7 Optimization plot for urea conversion from single pass testing in a single urea

electrolytic cell. The optimum conditions for obtaining maximum urea conversion

are anode flow rate 1.1 mL/min, cathode flow rate 120 mL/min, and cell voltage

1.6 V ........................................................................................................................................... 15

8 Optimization plot for the combination of hydrogen production and urea

conversion using single pass testing in a single urea electrolytic cell. The

optimum values are anode flow rate 17.9 mL/min, cathode flow rate 120

mL/min, and cell voltage 1.6 V ............................................................................................... 16

9 Photo of the GreenBox, which has 10 cells stacked together. The electrolyte

inlets to the anode and cathode sides are at the bottom and the outlets for the

solutions are placed on the top ............................................................................................... 16

10 Flow diagram of the home built electrolyzer test stand. The electrolyzer test

stand is connected to the GreenBox system .......................................................................... 17

11 Screenshot of the data collection program developed using the Camille software.

The program can record and control various parameters during electrolysis

experiment using the electrolyzer test stand ......................................................................... 19

12 Current-time response of the GreenBox during electrolysis with potassium

hydroxide solution. The electrolyte used in the experiment are 0.01 M KOH (pH

12), 0.1 M KOH (pH 13), and 1 M KOH (pH 14) .................................................................... 19

13 Plot of the hydrogen gas flow rate during the experimental time for various

concentrations of KOH solution in the GreenBox .................................................................. 20



ERDC/CERL TR-12-1 vi

14 Structure of the Platinum cluster (15 atoms) used in the Gaussian calculations.

The different locations for molecules to interact with the Pt cluster is labeled as

top site (T), bridge site (B), hcp hollow position (H), and fcc hollow position (F) ................. 24

15 Atomic structures of four different Pt clusters (Pt10, Pt15, Pt20, and Pt25). The Pt10

and Pt15 clusters have two layers of atoms, whereas Pt20 and Pt25 clusters have

three layers of atoms ................................................................................................................ 25

16 Structural orientation of urea molecule with nickel oxyhydroxide (NiOOH)

molecule. (a) Optimized structure of nitrogen coordinated urea on NiOOH, (b)

Optimized structure of oxygen coordinated urea on NiOOH, and (c) Optimized

structure for bridge-coordinated urea on NiOOH (Daramola, Singh, and Botte

2010) ......................................................................................................................................... 27

17 Cyclic voltammogram of Ni (0.5 mg/cm2) covered Ti electrode in urea solution.

The urea oxidation peak observed in the forward direction of the scan, whose

peak current and oxidation potential varies with the scan rate ........................................... 29

18 Urea oxidation peak potential as a function of scan rate. Cyclic voltammetry

using Ni covered Ti disk in urea solution (0.33 M urea + 5 M KOH) ................................... 30

19 Peak current for urea oxidation as function of the scan rate. The peak current (Ip)for urea oxidation is represented versus the square root of the scan rate (v ) .................... 31

20 Comparison of cyclic voltammograms of Ni and Ni-Rh electrodes in urea solution.

The electrolytes used for this comparison are blank solution (5 M KOH) and urea

solution (0.33 M urea + 5 M KOH) .......................................................................................... 33

21 Schematic diagram of the large sized Ni-Rh electrode used in the GreenBox ................... 33

22 Potentiostatic experiment of an electrolytic cell in blank and urea solution; the

anode and cathode is Rh deposited Ni mesh electrode ....................................................... 34

23 Plating apparatus designed for uniform distribution of Rh over Ni electrode,

showing (a) front view of the plating apparatus, and (b) side view of the

apparatus (which is immersed in plating solution) and the electrodes .............................. 35

24 SEM images of Rh plated Ni foil electrodes. The Rh was plated on the Ni foilelectrodes at cell voltages of (a) 1.43 V, (b) 1.51 V, (c) 1.61 V, (d) 1.70 V, and (e)

1.81 V ......................................................................................................................................... 37

25 XRD spectra of Rh deposited Ni foil. The presence of Rh(111), Rh(200), and

Ni(111) are observed in these spectra ................................................................................... 37

26 Urea oxidation charge as a function of Rh(111) peak intensity. The amount of

Rh(111) deposited have influenced the amount of urea molecules oxidized .................... 37

27 Urea oxidation charge as a function of Ni(111) peak intensity. The coverage of

Rh on Ni surface has influenced the amount of urea molecules oxidized ......................... 38

28 SEM images of the nickel-cobalt hydroxide electrodes. (a) Sample A, (b) Sample

B, (c) Sample C, (d) Sample D, (e) Sample E, and (f) Sample F ........................................... 39

29 Onset potential for urea oxidation as function of Co content in the bimetalliccatalyst. Sample C with 43% Co in the catalyst has the lowest onset potential ................. 40

30 XRD pattern of the SDS intercalated layered nickel hydroxide (Wang, Yan, and

Botte 2011) ............................................................................................................................... 41

31 AFM image of the exfoliated layered nickel hydroxide nanosheets. The lateral size

of the nanosheets ranged from a few hundred nanometers to a micrometer and

the thickness was around 1 nm (Wang, Yan, and Botte 2011) ............................................ 42

32 Comparison of cyclic voltammograms of the Ni(OH)2 nanosheets modified glassy

carbon electrode and bulk Ni(OH)2 powder modified glassy carbon electrode

(Wang, Yan, and Botte 2011) ................................................................................................... 43



ERDC/CERL TR-12-1 vii

Tables

1 Energy consumption for hydrogen production in different ammonia electrolysis

systems ........................................................................................................................................ 5

2 Energy consumption for hydrogen production in different urea electrolysis

systems ........................................................................................................................................ 9 3 Parameters and its levels used for the single cell testing ..................................................... 14

4 Experimental results from the single cell testing with hydrogen production rate

and urea conversion percentage............................................................................................. 14

5 Hydrogen flow rate from water electrolysis in the GreenBox ................................................ 20

6 Calculated reaction rate for the steps in the Oswin and Salomon ammonia

oxidation reaction mechanism ................................................................................................ 24

7 Adsorption energy for the molecules involved in ammonia electrolysis on Pt

cluster ........................................................................................................................................ 25

8 Binding energies for the molecules interaction with four different Pt clusters ................... 26

9 Kinetic information for the reaction steps involved in urea electrolysis .............................. 28 10 Cell voltage and current for the potentiostatic experiment of single cell in blank

and urea solution ...................................................................................................................... 34

11 Composition of deposition solution for nickel-cobalt hydroxide catalyst ............................. 39



ERDC/CERL TR-12-1 viii

Preface

This study was conducted for the Congress under a 2009 Department of

Defense Appropriations Bill, House of Representatives Report H.R. 3222,

110-434, page 268, and was monitored by Headquarters US Army Corps of

Engineers (HQUSACE) under P2 Project 321183, “Quiet, Low-Impact Al-

ternate Energy Technology — CERL-5.” The technical monitor was Kris

Gardner, ASA-ALT.

The work was performed by the Energy Branch (CF-E) of the Materials Di-

vision (CF), U.S. Army Engineer Research and Development Center –Construction Engineering Research Laboratory (ERDC-CERL). Franklin

Holcomb is Chief, CEERD-CF; L. Michael Golish is Chief, CEERD-CF. The

associated Technical Director was Martin J. Savoie , CEERD-CV-T. The

Deputy Director of ERDC-CERL is Dr. Kirankumar Topudurti and the Di-

rector of ERDC-CERL is Dr. Ilker R. Adiguzel.

CERL is an element of the U.S. Army Engineer Research and Development

Center (ERDC), U.S. Army Corps of Engineers. The Commander and Ex-

ecutive Director of ERDC is COL Kevin J. Wilson, and the Director of

ERDC is Dr. Jeffery P. Holland.



ERDC/CERL TR-12-1 1

1 Introduction

1.1 Background

The standard method of power generation in military base camp opera-

tions is via the use of diesel generators. Unfortunately, diesel generators

are often noisy and inefficient. Base camps could benefit from a “silent

camp™” operation, in which power is supplied via low-noise, low-impact

methods such as fuel cells. Fuel cells can run on a variety of fuels, but the

commercially available Proton Exchange Membrane (PEM) fuel cell re-

quires pure hydrogen, which can be commonly derived from the electroly-

sis of water.

Hydrogen can also be supplied by electrolyzing ammonia or urea, both of

which are found in wastewater. Of the two, ammonia has a higher hydro-

gen fraction of atomic mass (17.65%) than water. Two technologies devel-

oped by Ohio University can process wastewater (ammonia and urea) into

hydrogen, nitrogen, and clean water using renewable energy—the low en-

ergy consumption “ammonia and urea electrolyzers.” These two technolo-

gies may find direct application for the production of hydrogen for distrib-

uted power. It is anticipated that the hydrogen produced by these novel

processes may be used to run a fuel cell to generate electrical power, thusproviding a safe and convenient alternative to diesel generators. The tech-

nologies also have the potential for non-military applications (e.g., agricul-

tural wastewater, municipal wastewater, etc). The key feature of the am-

monia and urea electrolysis processes is the low energy consumption for

the production of hydrogen from ammonia and urea.

This work was undertaken to create an efficient and feasible process for

electrolyzing ammonia, specifically, by clarifying the electrolytic decompo-

sition pathways of ammonia (NH3) and urea (CO(NH2)2). These chemicals

are typically found in the excretions of all higher order vertebrates, and

can be readily found in feedlot run-off, municipal sanitary systems, etc.

The pathways for the electro-decomposition of these chemicals are rea-

sonably well understood. For example, several mechanisms had been pro-

posed in the literature for the oxidation of ammonia in alkaline media

(Oswin and Salomon 1963, Gerischer and Mauerer 1970, de Vooys et al.

2001), however, the final mechanisms have not been reported. Further-



ERDC/CERL TR-12-1 2

more, most of the mechanisms proposed in the literature do not account

for the adsorption of OH– ions on the catalyst, which has been reported to

affect the oxidation of ammonia (Botte 2004, Cooper and Botte 2006).

Also, it is important to know how to catalytically execute these processes,

using energetically efficient and mechanically robust electrodes employing

highly specific materials formulation coupled with innovative geometries.

There exist a large number of potential catalytic compositions and design

geometries. Further, the details of the evolved gases depend on the elec-

trode’s material and surface geometry parameters. A detailed correlation

between the evolved gases and these material parameters is required for

efficient use of this chemical decomposition process.

1.2 Objectives

The specific objectives of the Phase I of this project were to:

1. Determine kinetics and operating parameters required for the scale up of

the ammonia and urea electrolyzers.

2. Evaluate and optimize the performance of bench-scale prototypes (50 W)

with synthetic wastewater.

3. Provide the required information for a scale-up prototype (500 W).

1.3 Approach

The objectives of Phase I of the project were accomplished through the fol-

lowing tasks:

• Ammonia and urea electrolysis methods were compared with existing

water electrolysis technology. Improvements in ammonia and urea

electrolysis technologies and advancements in the catalyst develop-

ment were noted (Chapter 2).

• A 50 W GreenBox system, which was a scaled up version of the bench

scale single cell ammonia and urea electrolysis system, was built and

tested, in anticipation of the assembly of a 500 W electrolyzer in thePhase II of the project (Chapter 3).

• Reaction mechanisms for the electrochemical oxidation of ammonia

and urea were investigated considering both modeling and experi-

mental work for a rational design of the catalyst, and using molecular

modeling and prior experimental studies to advance reaction mecha-

nisms of ammonia and urea electrolysis (Chapter 4).



ERDC/CERL TR-12-1 3

• Catalysts with higher catalytic activity for efficient electrolysis were de-

veloped (Chapter 5).

• The potential for application of the ammonia and urea electrolysis

technologies in DoD programs and civilian applications were explored with special emphasis on wastewater remediation (Chapter 6).

1.4 Mode of technology transfer

This report will be made accessible through the World Wide Web (WWW)

at URL: http://www.cecer.army.mil



ERDC/CERL TR-12-1 4

2 Efficiency of Ammonia and

Urea Electrolysis

2.1 Ammonia electrolysis

Ammonia, which is found in wastewater, has a higher hydrogen fraction of

atomic mass (17.65%) than the water molecule. The electrolysis of ammo-

nia in an alkaline medium results in the formation of hydrogen gas at the

cathode electrode and nitrogen gas at the anode electrode. In this process,

the electrocatalyst used for the oxidation reaction of ammonia is platinum-

iridium (Pt-Ir) alloy, which is supported on a substrate (e.g., carbon fiber,metal, carbon paper, etc). The electrochemical reactions for the ammonia

electrolysis process are:

Anodic reaction

3 2 22 NH + 6 OH N + 6 H O + 2 e E = -0.77 V vs. SHE− −→ °Reaction 1

Cathodic reaction

2 22 H O + 2 e H + 2 OH E = -0.829 V vs. SHE− −→ °

Reaction 2

Overall reaction

Reaction 3

Thermodynamically, the electrochemical splitting of ammonia into hydro-

gen (H2) and nitrogen (N2) requires only 59 mV, whereas the thermody-

namic cell voltage for water electrolysis is 1.23 V. Theoretically, the elec-

trolysis of ammonia should consume 1.55 Wh to produce 1 g of hydrogen

gas. However, the energy consumption for different ammonia electrolysis

systems varies with different catalysts, support materials, and electrolytic

cell configurations. Table 1 lists the characteristics of three such systems,

along with the advantages and drawbacks found in each system.



Table 1. Energy consumption for hydrogen production in different ammonia electrolysis systems.

System Cell Voltage (V)

Energy

Consumption

(Wh/g of H2) Comments

1 Pt-Rh catalyst on carbon

fiber (Single cell)

0.33

(5 M NH3 and 50 °C)8.6

• Each experiment was performed in a closed cell with the

circulating electrolyte.

• All the electrodes are electrically connected with Ti wire,which is not a good electrically conducting material.

• The Pt-Rh catalyst is not the best catalyst for the electrolysis

of ammonia. Catalyst performance can be improved by

adding Ir.

Recommendations:

• Need better cell design for NH3 electrolysis.

• Use electrically conducting material such as Ni instead of Ti.

• Use Pt-Ir catalyst for ammonia electrolysis.

0.39

(5 M NH3 and 25 °C)10.0

0.5

(0.5 M NH3 and 50 °C)12.2

0.65

(0.5 M NH3 and 25 °C) 15.5

2 Pt-Ir catalyst on carbon

fiber paper

(~15.5 mg/cm2) (Biradar

2007)

0.36

(1 M NH3, single open cell and 55 °C)9.6

• The electrodes have high loading of Pt-Ir over carbon f iber

paper.

• The catalyst support material – carbon fiber paper is brittle.

• Ti frame and wires from the electrodes were used for

electrical contact.

Recommendations:

• Reduce the Pt-Ir catalyst loading.

• Change the carbon paper to a mechanically stronger and

electrically good conductor support material.

• Modify the cell design to improve easy flow of electrolyte and

gas removal without creating a significant back pressure.

0.46

(1 M NH3, single open cell and 25 °C)12.3

0.56

(1 M NH3, 9-cell stack and 55 °C) 15.4

0.63

(1 M NH3, 9-cell stack and 25 °C)17.3

3 Pt-Ir catalyst on carbon

fiber paper (12.4 mg/cm2)

(Boggs and Botte 2009)

0.52

(5 M NH3 solution, closed cell and

room temperature)

15



ERDC/CERL TR-12-1 6

2.2 Recommendations regarding ammonia electrolysis

Of the three systems described, System 1 offers the most promising oppor-

tunity to extend the catalyst composition by using a better electronically

conductive tab for the electrode. Phase II of this project will continue to

focus on adapting the electroplating capabilities of System 1, which con-

sumes the least energy in the production of hydrogen, through the use of

more highly electronically conductive tabs (e.g., nickel tabs).

The experimental data from a 9-cell stack electrolyzer for ammonia

(Figure 1), was used to illustrate the energy consumption required to pro-

duce 1 g of hydrogen gas. The electrodes in this electrochemical cell had a

high loading of Pt-Ir over carbon fiber paper (System 2 in Table 1). The

ammonia electrolysis in the 9-cell stack electrolyzer was operated at 4.5amps, which resulted in a cell voltage of 0.63 V at 25 °C and produced

0.164 g of hydrogen per hour (System 2 in Table 1). In other words, the 9-

cell stack ammonia electrolyzer required 17.3 Wh of electrical energy to

produce 1 g of H2.

Unfortunately, the system showed a significant ohmic overpotential due

to: (1) the use of titanium tabs, (2) electrical losses through the connecting

wires, and (3) high electrical resistance in the contact between the wires

and the tabs. To overcome this, Phase II of this project will need to include

a significant study of ohmic overpotential minimization through the tabsand electrical connections through the construction of the 500 W urea

electrolyzer.

By comparison, water electrolysis consumes more energy than does am-

monia electrolysis to produce the same amount of hydrogen gas. The Na-

tional Research Council and National Academy of Engineers report on the

“hydrogen economy” (NRC & NAE 2004) states that a commercial water

electrolyzer requires 1050 kW of power to produce 8290 scf of H2

(21103.7 g of H2) per hour, or 49.75 Wh/g. An ammonia electrolyzer, which uses 17.3 Wh to produce the same result, is 65.1% more efficient

than a commercial water electrolyzer.



ERDC/CERL TR-12-1 7

Figure 1. Experimental set up of the 9-cell stack of the ammonia electrolytic cell (AEC). During

ammonia electrolysis, the collection chambers at the cathode and anode side accumulated

the hydrogen and nitrogen gases, respectively.

2.3 Urea electrolysis

Urea is used as fertilizer and is also found in the liquid waste of human

and other animals. The electrochemical oxidation of urea can remediate

wastewater and also produce hydrogen gas, a high-value product. The ac-

tive catalyst for electrochemical oxidation of urea is nickel. (In contrast,

the active catalyst for ammonia electrolysis is platinum.) The electrochem-

ical reactions for the urea electrolysis are:

Anodic reaction

2 2 2 2 2CO(NH ) + 6 OH N + 5 H O + CO + 6 e E = -0.46 V vs. SHE− −→ ° Reaction 4

Cathodic reaction

2 26 H O + 6 e 3 H + 6 OH E = -0.829 V vs. SHE− −→ ° Reaction 5

Overall reaction

Reaction 6

The electrochemical oxidation of urea thermodynamically requires only

0.369 V and produces hydrogen, nitrogen, and carbon dioxide gases. (By

contrast, water electrolysis requires 1.23 V.) Table 2 lists two electrolytic

cells with different kinds of catalyst and electrode configuration for the

production of hydrogen gas.



ERDC/CERL TR-12-1 8

2.4 Recommendations for Phase II (urea electrolysis)

Overall, development of the Ni-Rh catalyst (Case 2), with improvement on

preferentially depositing Rh(111) over the Ni electrodes, will be performed

in Phase II of this project. Nano-sized Ni-Rh electrode could increase the

current density for the urea electrolysis. There are also promising signs

from the other catalysts, Ni-Co hydroxide (Case 3) and Ni(OH)2

nanosheets (Case 4). Phase II will perform more systematic investigations

to develop nano-sized catalysts along the lines of Ni(OH)2 nanosheets.

The single urea electrolytic cell with Ni-Rh electrodes (Case 2 listed in Ta-

ble 2), of which the lower half of the Ni electrode is covered with Rh, was

subjected to 1.6 V, and the urea solution (0.33 M urea + 5 M KOH) was

circulated once through the anode side. This electrolytic cell generated hy-drogen gas at a rate of 0.001 g/minute with an average current of 1.311 A.

This single cell consumed 34.96 Wh to produce 1 g of hydrogen, at least

29.7% less energy than water electrolyzer, which consumed 49.75 Wh to

produce 1 g of hydrogen.



Table 2. Energy consumption for hydrogen production in different urea electrolysis systems

Systems

Cell Voltage

(V)

Energy consumption

(Wh/g of H2) Comments

1. Nickel oxyhydroxide (NiOOH) modified Ni electrode

(2.5 mg/cm2) (Boggs, King, and Botte 2009)

1.4 37.5 • A 2 cm x 2 cm size electrode was st udied.

• Ni sheet was the catalyst support material and electrical contact of the electrode.

Recommendations:

• Increase the surface area of the catalyst and the support material.• Use bimetallic or Ni based alloy catalyst to overcome surface blockage due to CO.

2. Rh deposited Ni mesh electrode (0.5 mg/cm2 of

Rh covering lower half of Ni mesh electrode)

1.6 34.96 • Room temperature operation.

• Single pass on the anode side of the electrolytic cell.

• Various parameters of the electrolysis process were not optimized.

• The particle size was larger and the crystal plane of Rh was not well defined.

Recommendation:

• Develop nano-sized Ni-Rh catalyst and deposit preferentially Rh(111).

• All the operating parameters for the urea electrolyzer have to be optimized.

3. Ni-Co hydroxide Promising performance to

be measured in Phase II

• Bench scale experiments with small electrodes.

• Gas collection and its analysis have to be performed.

Recommendations:

• Systematic experimentation to collect and measure the anode and cathode gases.

• Scale up of the 43% (at.wt.) Co added Ni(OH)2 catalyst for GreenBox application.

4. Ni(OH)2 nanosheet Promising performance to

be measured in Phase II

• Nanosized catalyst for urea electrolysis.

• Uses Teflon binding material to attach the nanosheets to an electrically conducting substrate

• Very high surface area available for the electrocatalytic activity

Recommendations:

• Investigate on adherence of the nanosheet to the conducting material without Teflon binder.

• Plating of other catalyst over the nanosheets so as to use nanosheets as support material.

• Scale up of the nanosheets to the GreenBox size electrodes.



ERDC/CERL TR-12-1 10

3 Test of 50 W GreenBox

Part of the effort in Phase I of the project focused on the demonstration of

the scale up of the urea electrolysis process from a single cell bench scale

into a urea electrolyzer stack. Knowledge obtained from the scale up pro-

cess was used in the design of the 500 watts electrolyzer, which is part of

Phase II of the project.

3.1 Cell design

In the electrochemical oxidation of urea, nickel is the active material.

(More specifically, Ni2+/Ni3+ redox couple is the required electrocatalyst.)During the urea electrolysis, carbon-based compounds or intermediates

were formed on the electrode surface, which have been observed to block

or interfere with the nickel active sites. To address this problem, rhodium

(Rh) was deposited on the Ni electrodes to help in preventing the build up

or surface blockage of Ni sites by carbon-based compounds, which could in

turn maintain or enhance the reaction rate for electrochemical oxidation

of urea.

Figure 2 shows the initial design for a large size single cell to perform urea

electrolysis. The cell has end plates to hold the electrode and a membrane

is used to separate the anode side from the cathode side. The membrane is

held in place by two gaskets. This cell has a provision to fill the electrolyte

on each side of the electrode by using the inlet and outlet provided at the

top and bottom of each end plate. The single cell can perform the urea

electrolysis experiment under static or flow conditions.

This design of the single cell is for planar electrodes, where both anode

and cathode electrodes are nickel (Ni) sheets, but where each has different

amounts of rhodium (Rh) loadings. The electrode dimension was 7 x 7-in.and Rh was deposited only in the lower half of the electrode. The anode

electrode had Rh loading of 0.5 mg/cm2 and the cathode electrode was

loaded with 1 mg/cm2 of Rh. After an hour of testing the single cell shown

in Figure 2, the oxidation current for urea at 1.6 V was only 1.16 A. This

translates to urea oxidation current density of only 3.7 mA/cm2. To im-

prove the electro-oxidation current for urea, the electrode design was

modified to an extended oval shape (Figure 3).




Figure 2. Exploded view of the first large scale single cell designed to perform urea

electrolysis. The cell comprises of end plate (A), anode electrode (B), gasket sheet (C),

membrane (D), and cathode electrode (E).

Figure 3. Diagram of the new electrode for urea electrolysis. This electrode (Ni mesh #40) is

the base material for both anode and cathode electrodes of the GreenBox.

Measured between its top and bottom hemisphere, the new Ni mesh elec-

trode is 6-in. wide and 9-in. long. The Rh catalyst was deposited on the low-

er half of the Ni mesh electrode with 0.5 mg/cm2 loading for the anode elec-

trode and 1 mg/cm2 loading on the cathode electrode. A single cell was

developed using this new electrode design. Figure 4 shows an exploded view

of the design. The membrane used to separate the anode and the cathode

side of the single cell is held in place by a O-ring located in the end plate.This O-ring also provides a good seal for liquid and gases. There were no

leaks observed from the single cell during urea electrolysis. The advantage

in using the new Ni mesh electrodes is the design of the electrode. The ex-

tended oval shape of the electrode along with wire mesh provides larger sur-

face area than the rectangular electrode shown in Figure 2. The extended

oval shape of the electrode is also grooved in the inner side of the end plate

(Figure 4), which facilitates easy removal of gas bubbles from the cell and

prevents a build-up of back pressure inside the electrolytic cell.

A B CD

E

A

C




Figure 4. Sectional exploded view of the single cell assembled based on the new electrode

design. The anode electrode (A) and the cathode electrode (C) are separated by a membrane

(B). The electrodes are covered to the external atmosphere by an end plate (D).

3.2 Single cell testing

The extended oval shaped Ni mesh electrodes with Rh loading were as-

sembled in the new single cell. Each single cell was tested for urea electrol-

ysis under a condition of flow. The electrolyte used in both the anode and

the cathode sides was 0.33 M urea + 5 M KOH. The urea concentration

chosen in the electrolyte for the experiments reflects the average amount

of urea observed in human urine, 20 g/L/day. Figure 5 shows the flow dia-

gram of a single cell testing setup. Testing was done to evaluate a few pa-

rameters on the production of hydrogen gas, and to evaluate the percent-

age of urea conversion. The urea conversion rate was determined for the

single cell system where the anode electrolyte flowed through the urea

electrolytic cell once (in a single pass).

Figure 5. Flow diagram of the single cell testing set up. The cathode electrolyte is circulated

through the cathode side of the cell whereas the anode electrolyte is passed through the

anode side only once (single pass) to evaluate the urea conversion percentage.

A

B

D

C




As Figure 5 shows, the cathode solution is pumped into the cathode side of

the single cell from its reservoir. The spent cathode solution from the urea

electrolytic cell passes through a gas separating column, where the hydro-

gen gas is separated from the urea solution, and from where the solutionflows back into the cathode reservoir. The separated hydrogen gas is col-

lected in a gas column using the water displacement principle. Similar to

the cathode side, the anode solution is pumped into the anode side of the

urea electrolytic cell from its reservoir. The spent anode solution also goes

through a gas separating column. Since these experiments used a single

pass for the anode solution, the anode effluent was collected for urea anal-

ysis. The separated anode gases were also collected in a gas collecting col-

umn. The volume of gases collected in the anode and cathode sides was

measured over the testing duration.

The single cell test with single pass on the anode side evaluated these pa-

rameters: cell voltage, anode electrolyte flow rate, and cathode electrolyte

flow rate (Table 3). Note that Minitab® software*

Table

was used to prepare the

Box-Benkhen surface response experimental design and also to statistical-

ly analyze the results.

4 lists the results of 15 single cell tests. Note that the anode flow rate

in Tables 3 and 4 represent readings of the pump settings, and not the

measured flow rate of the electrolyte. The exact flow rate corresponding to

the pump settings was found separately, and the anode electrolyte flow

rate relating the reading values was used in the statistical analysis. The

urea concentration in the anode effluent was determined using the spec-

trophotometric method. The urea conversion percentage was calculated

from the ratio of the urea concentration in the anode effluent to the anode

solution.

The data in Table 4 show that Experiment No. 5 shows zero hydrogen gas

produced, at 1.4 V with the cathode electrolyte flow rate of 75 mL/min and

anode flow reading of 5, is an anomalous value. The amount of hydrogengas collected in the column during that experiment was very minimal and

the volume of the hydrogen gas in the column was not sufficient enough to

measure.

* Minitab® Software for Quality Improvement, http://www.minitab.com




Table 3. Parameters and its levels used for the single cell testing.

Parameter Low Medium High

Potential (V) 1.4 1.5 1.6

Anode Flow Reading 5 18.75 32.5

Cathode Flow Rate (mL/min) 30 75 120

Table 4. Experimental results from the single cell testing with hydrogen production rate and

urea conversion percentage.

Run

Order

Std

Order

Pt

Type Blocks Anode Flow

Cathode

Flow

(mL/min)

Potential

(V)

Hydrogen

(g/min)

Urea

Conversion

(%)

1 1 2 1 5 30 1.5 0.0002 23.51

2 9 2 1 18.75 30 1.4 0.00002 29.73

3 10 2 1 18.75 120 1.4 0.0003 25.83

4 7 2 1 5 75 1.6 0.0007 25.835 5 2 1 5 75 1.4 0 26.12

6 13 0 1 18.75 75 1.5 0.0003 22.08

7 14 0 1 18.75 75 1.5 0.0002 2.21

8 2 2 1 32.5 30 1.5 0.0002 24.55

9 8 2 1 32.5 75 1.6 0.0009 23.89

10 12 2 1 18.75 120 1.6 0.001 18.68

11 6 2 1 32.5 75 1.4 0.00002 3.64

12 11 2 1 18.75 30 1.6 0.0007 2.77

13 3 2 1 5 120 1.5 0.0002 20.44

14 15 0 1 18.75 75 1.5 0.0003 24.39

15 4 2 1 32.5 120 1.5 0.0002 11.29

A surface response analysis of the results listed in Table 4 was done to de-

termine the optimum conditions to maximize hydrogen production

(Figure 6). Figure 6 shows that if the anode electrolyte flows at a rate of 7.8

mL/min with cathode solution flowing at 120 mL/min and at a cell voltage

of 1.6 V, the single cell should be able to produce the largest volume of hy-

drogen gas.

The urea conversion from the single pass urea electrolysis in a single cell was also studied using the surface analysis and the optimization plot for

maximizing urea conversion (Figure 7). The maximum percentage of urea

can be converted into its gases when the anode electrolyte flow rate is

maintained at 1.1 mL/min with the cathode solution flowing at 120

mL/min and with a cell voltage of 1.6 V applied to the single cell.




Figure 6. Optimization plot for hydrogen production in a single pass testing of the single urea

electrolytic cell. Optimum values for obtaining maximum hydrogen production are anode flow

rate 7.8 mL/min, cathode flow rate 120 mL/min, and cell voltage 1.6 V.

Figure 7. Optimization plot for urea conversion from single pass testing in a single urea

electrolytic cell. The optimum conditions for obtaining maximum urea conversion are anode

flow rate 1.1 mL/min, cathode flow rate 120 mL/min, and cell voltage 1.6 V.

The optimum value for the anode flow rate changes if both hydrogen gen-

eration rate and urea conversion percentage are considered in combina-

tion. Figure 8 shows the optimization plot for the combination of the hy-

drogen production and urea conversions, in which the anode flow rate is

increased to 17.9 mL/min with other two parameters are maintained at the

same value (of 120 mL/min for cathode flow rate and 1.6 V for cell volt-

age).

3.3 50 W GreenBox testing

The GreenBox system was designed to remove urea from wastewater and

produce hydrogen gas. Each urea electrolytic cell as shown in Figure 4 was

assembled together into a stack of 10 cells to form the first prototype

GreenBox system (Figure 9). The GreenBox is expected to consume 50 W

of electrical energy.

CurHigh

Low0.87307D

Optimal

d = 0.87307

Targ: 0.0010Hydrogen

y = 0.0010

0.87307

DesirabilityComposite

1.40

1.60

30.0

120.0

5.0

32.50Cathode Potentia Anode FL

[24.7222] [120.0] [1.60]




Figure 8. Optimization plot for the combination of hydrogen production and urea conversion

using single pass testing in a single urea electrolytic cell. The optimum values are anode flow

rate 17.9 mL/min, cathode flow rate 120 mL/min, and cell voltage 1.6 V.

Figure 9. Photo of the GreenBox, which has 10 cells stacked together. The electrolyte inlets

to the anode and cathode sides are at the bottom and the outlets for the solutions are placed

on the top.

Testing of the GreenBox required a well controlled and monitored test

stand so a test stand was built at the Center for Electrochemical Engineer-

ing Research (CEER) to regulate electrolyte flow into each cell, and to

monitor pressure, temperature, pH, and gas flow rate. Figure 10 shows a

flow diagram of the electrolyzer test stand connected to the GreenBox. The

liquid/gas separator for the cathode and anode side is used as the reservoir

for the corresponding electrolyte. The cathodic side pump draws the elec-

trolyte from the cathode liquid/gas separator and pumps the solution into

the cathode side of the GreenBox system via a manifold. The flow rate of

the electrolyte in the cathode side is maintained and regulated by the liq-

uid flow meter (LFM) and the flow control valve (FC).




Green Box

Anode PumpCathode Pump

Hydrogen Storage

LFM

L F M

T

P T

P

Dr ai n a g e

V al v e

Dr ai n a g e

V al v e

TP T

P

FC

FC

GFM

GFM

Cathode L/G Separator

Inlet Flow Valve

Anode L/G Separator Inlet Flow Valve

Drainage Valve

C a t h o d e L i q u i d / G a s S e p a r a t o r

A n o d e L i q u i d / G a s S e p a r a t o r

D e s i c c a t o r

D e s i c

c a t o r

C a t h o d e S i d e

A n o d e S i d e

FeedTank

Vent

Vent

pH – pH Meter T - Thermocouple

P – Pressure Gauge

LFM – Liquid Flow Meter GFM – Gas Flow Meter

FC – Flow Control Valve

pH

pH

DC Power Supply

Figure 10. Flow diagram of the home built electrolyzer test stand. The electrolyzer test stand

is connected to the GreenBox system.

The inlet and outlet temperature and pressure of the cathode electrolyteare monitored. The pH of the cathode electrolyte is monitored on the out-

let side. The cathode electrolyte exiting the GreenBox system carries hy-

drogen gas, which is separated in the liquid/gas separator. The hydrogen

gas is collected in a water column (labeled as hydrogen storage in the flow

diagram). Before venting, the hydrogen gas passes through a desiccator to

remove moisture and then through a gas flow meter.

The test stand components on the anode side of the electrolyzer are similar

to those on the cathode side described above. An additional feature included

in the anode side is a drainage valve at the tube line coming from the end of anode liquid/gas separator so that the anode effluent from the anode side of

the GreenBox can be drained out during single pass experiments for testing

and monitoring. In the single pass experiments, the anode pump draws the

electrolyte from a feed tank. The GreenBox is supplied with cell voltage by a

DC power supply unit. The controls on the flow meter, thermocouple, pres-

sure transducers, and pumps are digitally regulated through data acquisi-

tion unit, which is in turn observed by a program developed in Camille

software. The program is capable of digitally controlling, monitoring, and




recording the parameters mentioned above along with cell voltage and cur-

rent for all the 10 cells. Figure 11 shows a screenshot of the program devel-

oped to record and control the parameters.

Preliminary experiments using the GreenBox were performed with the

help of the electrolyzer test stand. These experiments used potassium hy-

droxide solutions of different concentrations to generate a baseline values

before using urea solutions in the GreenBox. In all the experiments, the

electrolyte solution was constantly circulated with an anode flow rate of 80

mL/min and a cathode flow rate was maintained at 1200 mL/min. These

flow rate values reflect the optimum flow rate values determined from the

single cell testing for maximizing hydrogen gas production. The electrolyte

concentrations used in the preliminary experiments until now were 0.01,

0.1, and 1 M KOH, which corresponds to pH value of 12, 13, and 14. Thecell voltage for all the 10 cells was varied between 1.5 and 1.575 V for these

experiments. Figure 12 shows the current-time response for the GreenBox

over the period of entire experiment (3 hrs).

The individual cell was designed to have a maximum current of 3.2 A at 5

M KOH concentration. The current observed during the baseline experi-

ments is low because it is mainly due to side reactions associated with the

phase transition of the anode and parasitic losses.

The electrolysis of different concentrations of KOH solution in the

GreenBox produces the hydrogen gas and the flow rate of the hydrogen gas

over the 3-hr experiment (Figure 13). The hydrogen gas seen on the cath-

ode side could be due to the slight water oxidation at these cell voltages for

the different concentrations of KOH solution. It is hypothesized that the

minor hydrogen gas produced is mainly due to the oxidation of nickel to

its active form (Boggs, King, and Botte 2009), and not to a continuous

process such as urea or water electrolysis.

The data in Figure 13 indicate that the production of hydrogen gas washigher at cell voltage of 1.575 V than at 1.5 V for pH 12 and 13 solutions,

which was expected. Table 5 lists the average hydrogen flow rate at differ-

ent KOH solutions and cell voltages. At cell voltage of 1.575 V, the average

flow rate for the production of hydrogen increased more than 10 mL/min

with one pH unit increase in the KOH solution. However, the hydrogen

production rate was similar for KOH solutions of pH 13 and 14 at a cell

voltage of 1.5 V.




Figure 11. Screenshot of the data collection program developed using the Camille software.

The program can record and control various parameters during electrolysis experiment using

the electrolyzer test stand.

Figure 12. Current-time response of the GreenBox during electrolysis with potassium

hydroxide solution. The electrolyte used in the experiment are 0.01 M KOH (pH 12), 0.1 M

KOH (pH 13), and 1 M KOH (pH 14).

0

0.5

1

1.5

2

2.5

3

0 20 40 60 80 100 120 140 160 180 200

T o t a l C u r r e n t ( A )

Time (min)

pH 12 at 1.575 V

pH 13 at 1.5 V

pH 13 at 1.575 V

pH 14 at 1.5 V




Figure 13. Plot of the hydrogen gas flow rate during the experimental time for various

concentrations of KOH solution in the GreenBox.

Table 5. Hydrogen flow rate from water electrolysis in the GreenBox.

Electrolyte

solution pH

Average hydrogen flow rate (mL/min)

1.5 V 1.575 V

12 – 16.32

13 10.44 27.88

14 9.81 –

No further experiments were conducted with the custom-built test stand

on the 50 W GreenBox because of the many engineering problems and is-

sues that developed while running the baseline experiments (i.e., proper

functioning of the pumps, flow meters, and gas separation columns). The

test stand is presently being redesigned to ensure that the working of each

component is accurately assessed and calibrated. These issues have ex-

tended the evaluation and optimization of the 50 W GreenBox into Phase

II of this project.

3.4 Recommendations for Phase II (GreenBox)

It is recommended that Phase II of this project include:

• further experiments with the GreenBox, which include urea solution

containing 0.33 M urea

• further experiments that consider other parameters related to the urea

electrolysis in the GreenBox system.

0

20

40

60

80

100

120

140

0 20 40 60 80 100 120 140 160 180 200

H y d r o g e n F l o w r a t e ( m L / m i n

)

Time (min)

pH 12 at 1.575 V

pH 13 at 1.5 V

pH 13 at 1.575 V

pH 14 at 1.5 V




4 The Reaction Mechanisms

4.1 Electro-oxidation of ammonia

Two different reaction mechanisms for the electrochemical oxidation of

ammonia in alkaline media have been proposed, one by Oswin and Salo-

mon (1963), and one by Gerischer and Mauerer (1970). The reaction steps

proposed by Oswin and Salomon, in 1963, for the oxidation of ammonia

are:

3 3 NH + M MNHReaction 7

3 2 2MNH + OH MNH + H O + e− −

Reaction 8

2 2MNH + OH MNH + H O + e− − Reaction 9

2MNH + OH MN + H O + e− − Reaction 10

2 2MN + MN M N Reaction 11

2 2 2M N N + 2 M Reaction 12

where:

M represents the catalyst material

The electrochemical reactions takes place on the catalyst surface.

Gerischer and Mauerer (1970) proposed a reaction mechanism for the

ammonia electro-oxidation with inclusion of extra reaction steps to the

ones suggested by Oswin and Salomon:

3 3 NH + M MNHReaction 7

3 2 2MNH + OH MNH + H O + e− −

Reaction 8

2 2MNH + OH MNH + H O + e− − Reaction 9

2MNH + OH MN + H O + e− − Reaction 10

2 2MN + MN M N

Reaction 11

x y 2 2 (x+y)MNH + MNH M N H Reaction 13

2 2 (x+y) 2 2 2M N H + (x+y) OH M N + (x+y) H O + (x+y) e− − Reaction 14

2 2 2M N N + 2 MReaction 12

The steps in the Gerischer and Mauerer reaction mechanism follow the

same reactions listed by Oswin and Salomon except for those shown in

Reactions 13 and 14 . According to Gerischer and Mauerer, additional re-

actions and intermediates are formed during the electrochemical oxidation




of ammonia. The secondary reactions involves other intermediates such as

NHx and NH y (x and y = 1 or 2). These NHx and NH y intermediate mole-

cules can combine to form N2H2, N2H3, and N2H4 molecules (Reaction 13),

which eventually react with OH–

ions to form adsorbed N2 and water mol-ecules (Reaction 14).

The development of a highly efficient electrocatalyst for the oxidation of

ammonia requires a thorough understanding of different reaction steps

involved in the mechanism. Molecular modeling of the reactions steps

provides information on the structural changes of each molecule with the

catalyst surface. Additionally, the reaction rates of the different reaction

steps mentioned in both mechanisms can be calculated from molecular

modeling. In the molecular modeling, geometric optimization, frequency

calculations, and electronic structure calculations are prepared using theDensity Functional Theory (DFT) method using the Gaussian 03 software

(Gaussian, Inc. 2003). The basis set used for the calculation consists of the

Los Alamos National Laboratory double-ζ quality (LANL2DZ) (Wadt and

Hay 1985a, Hay and Wadt 1985, Wadt and Hay 1985b) for platinum and 6-

311++g** (Hehre, Schlayer, and Pople 1986) for nitrogen, hydrogen, and

oxygen atoms.

The LANL2DZ basis set includes a Los Alamos Effective Core

Pseudopotential with the double zeta basis set for the outer electrons; 6-

31g* is a basis set with a single polarization function while 6-311++g** is a

more complete basis set with double polarization (**) and diffuse func-

tions (++). The smaller basis set as shown in the article was able to ap-

proximate the bond lengths and bond angles for urea in comparison to

other basis sets. The level of theory employed in these calculations is a hy-

brid B3LYP correlation Hamiltonian functional (Becke 1993). The B3LYP

hybrid functional involves the use of a percentage of the Hartree Fock en-

ergy in combination with the Vosko, Wilk, and Nusair functional III for

local correlation and the Lee, Yang, and Parr functional for non-local cor-

relation. The transition state theory was employed in determining the re-action rate for all the steps mentioned in the two reaction mechanisms. It

uses classical mechanics in determining partition functions and, based on

the partition functions, the rate constant of the reactions can be deter-

mined (Santen and Niemantsverdriet 1995) as shown in:




#

b i

n

j

j = 1

= expk T q E

k h RT

q

− ∏

Eq. 1

where:

k = the rate constant (s-1 for unimolecular reactions and L mol-1 s-1

for bimolecular reactions)

q# = the partition function of transition states

q j = the partition functions of the reactants

E i = the difference in zero point energies of the reactants and

transition state structures (J mol-1)

k b = Boltzmann’s constanth = Planck’s constant (J s)

T = 298 K

R = universal gas constant (J mol-1 K -1).

The electrocatalyst used for ammonia oxidation is platinum (Pt) so all the

calculations regarding the ammonia oxidation mechanism are based on a

platinum catalyst surface. A 15 Pt atom cluster was developed (Figure 14)

to perform transition state calculations to determine the reaction rates for

the individual steps in the Oswin and Salomon mechanism.

Based on the geometric optimization, the preferred sites for the adsorption

of an ammonia (NH3) molecule is the top site (T) of the Pt cluster; the NH2

intermediate molecule binds at the bridge (B) location; NH and N inter-

mediates prefer the fcc hollow location; and the hydroxide ion (OH–) also

prefers to bind on the top location (T) of the Pt cluster. The reaction rates

for the Oswin and Salomon mechanism steps were calculated using the

transition theory (Table 6). The slowest rate of the reaction indicates that

reaction step to the rate determining step (RDS), which can help to explain

the interactions of the surface atoms of the Pt cluster with the intermedi-ate molecules. Reaction step 4 in Table 6 indicate that the combination of

nitrogen atom on the surface of Pt catalyst to form a nitrogen molecule

(N2) is the rate determining step as its ‘k’ value is only 1.40 x 10-21 s-1.




Figure 14. Structure of the Platinum cluster (15 atoms) used in the Gaussian calculations.

The different locations for molecules to interact with the Pt cluster is labeled as top site (T),

bridge site (B), hcp hollow position (H), and fcc hollow position (F).

Table 6. Calculated reaction rate for the steps in the Oswin and Salomon ammonia oxidation

reaction mechanism.

Reaction steps Rate constant, k (s-1)

13 2 2PtNH + OH PtNH + H O + e− −

1.34 x 108

22 2PtNH + OH PtNH + H O + e− −

2.17 x 106

32PtNH + OH PtN + H O + e− −

7.33 x 102

42 2PtN + PtN Pt N 1.40 x 10-21

The intermediate molecules such as NH2, NH, and N as well as NH3 ad-

sorb on the Pt surface so as to proceed with the electrochemical reaction.

The structure of the interaction between the molecule and the Pt surface

was optimized using the Gaussian software. Table 7 lists the binding ener-

gy or adsorption energy values calculated with and without the inclusion of

spin quantum numbers to both Pt atoms and the interacting molecule.

Each element has certain number of unpaired electrons, which contributes

to its spin quantum number. According to the data in Table 7, the addition

of spin quantum numbers in the calculations has brought the adsorption

energies closer to the values from Pt slab (bulk) model reported in the lit-

erature.

To reproduce the Pt bulk surface for the Gaussian calculations, the size of

the Pt cluster was increased to 15, 20, and 25 atoms. This extension of the

Pt surface with more atoms will imitate the periodic structure of the bulk

Pt. The 15-atom Pt cluster is made of two layers of atoms, similar to the 10-

atom Pt cluster. The 20- and 25-atom Pt cluster requires three layers to fit

all the atoms. Figure 15 shows the top and side view of the four Pt clusters

used in the Gaussian calculations.




Table 7. Adsorption energy for the molecules involved in ammonia electrolysis on Pt cluster.

Molecule

Adsorption Energy (kJ/mol)

Adsorbed NH3 Adsorbed NH2 Adsorbed NH Adsorbed N

Illustrate

Cluster (no spin) –86 –513 –483 –980

Cluster (spin) –65 –167 –259 –322

Slab model (ref) –68 –298 –387 –449

P t10 cluster Pt15 cluster

P t20 cluster Pt25 cluster

Figure 15. Atomic structures of four different Pt clusters (Pt10, Pt15, Pt20, and Pt25). The Pt10

and Pt15 clusters have two layers of atoms, whereas Pt20 and Pt25 clusters have three layers

of atoms.

The Pt clusters were used in the geometric optimization for the molecules

involved in the ammonia electro-oxidation. Table 8 lists the binding ener-

gy values for the molecules (NH3, NH2, NH, N, OH–, and H2O) with Pt

clusters such as Pt10, Pt15, Pt20, and Pt25. The data in Table 7 indicate that

the binding energy values for the Pt20 and Pt25 cluster were closer to the

slab model. The inclusion of OH– ion and water molecule is to improve the

Gaussian calculations to resemble the real environment during the ammo-

nia electrolysis.




Table 8. Binding energies for the molecules interaction with four different Pt clusters.

Molecule

Binding Energy (kJ/mol)

NH3 NH2 NH N OH H2O

Pt10 -56 -150 -274 -354 -178 -17

Pt15 -65 -167 -259 -322 -157 -19

Pt20 -71 -152 -248 -325 -156 -22

Pt25 -75 -179 -314 -350

The increasing binding energy values for the three layered cluster was ex-

pected. The top layer of the Pt20 cluster is larger than that of the Pt25 clus-

ter, which can be used to calculate multi-molecule adsorption scenarios.

Moreover, the time required to calculate using the Pt25 cluster is too long

compared to the Pt20 cluster, so the Pt20 cluster will be used for the future

Gaussian calculations. From a comparison of the binding energies for the

N based molecules with all of the four Pt clusters, it is obvious that N atom

has higher affinity with the Pt surface than with the NH3 molecule. When

the water molecule is considered along with the ammonia, which is ex-

pected during the electrolysis process, the preferential adsorption of the

water’s OH– ion on the Pt surface tends to block or exclude the NH3 mole-

cule. This reaction pathway information is important for further Gaussian

calculations as well as for experimental investigations.

4.2 Recommendations for Phase II

It is recommended that Phase II of this project:

• calculate the reaction rates for the steps in the Oswin and Salomon

mechanism Using the updated Pt20 cluster

• perform the geometry optimization for the N2H y molecules (N2H2,

N2H3, and N2H4) with the Pt20 cluster

• calculate the reaction steps found in the Gerischer and Mauerer mech-

anism using transition state theory with the Pt20 cluster, and determine

the reaction rates for each step

• experimentally verify the presence of the intermediates and confirm

the molecular model using in-situ spectroscopic methods such as Ra-

man, Fourier transform infrared (FT-IR), and x-ray diffraction (XRD)

spectroscopy.

4.3 Electro-oxidation of urea

The electrochemical oxidation of urea in alkaline media requires an Ni

based catalyst, which has been nickel hydroxide (Ni(OH)2) and nickel

oxyhydroxide (NiOOH). The decomposition of urea to ammonia is known




as the hydrolysis of urea, which happens in the presence of a urease en-

zyme (Estiu and Merz 2004a, Estiu and Metz 2004b, Estiu and Merz

2007, Fearon 1926). There are experimental and theoretical studies per-

formed on the hydrolysis mechanism (Musiani et al. 2001; Barrios andLippard 2000; Benini 1999; Alexandrova and Jorgensen 2007; Estiu,

Suarez, and Merz 2006), but no established publication was found on the

electrochemical oxidation of urea in alkaline media.

The investigation of molecular modeling on the urea electro-oxidation was

done similarly to the study of the electrochemical oxidation of ammonia

mechanism. DFT calculations were performed on the NiOOH catalyst and

other molecules involved in the reactions. The level of theory used in the

calculations is a hybrid B3LYP correlation Hamiltonian functional (Becke

1993) with the basis set consists of Los Alamos National Laboratory of double-ζ quality (LANL2DZ) (Wadt and Hay 1985a, Hay and Wadt 1985,

Wadt and Hay 1985b) for nickel and 6-31 g* (Hehre, Schlayer, and Pople

1986) for carbon, nitrogen, hydrogen, and oxygen atoms. The reaction

steps in the mechanism were studied using the transition state theory,

where the rate constant for the individual steps are determined (Santen

and Niemantsverdriet 1995) using Equation 1.

Figure 16 shows the optimized structure for the interaction of urea mole-

cule with the NiOOH structure. The orientation of the urea molecule to-

wards the NiOOH catalyst will determine the reaction pathway for urea

electro-oxidation.

Figure 16. Structural orientation of urea molecule with nickel oxyhydroxide (NiOOH) molecule.

(a) Optimized structure of nitrogen coordinated urea on NiOOH, (b) Optimized structure of

oxygen coordinated urea on NiOOH, and (c) Optimized structure for bridge-coordinated urea

on NiOOH (Daramola, Singh, and Botte 2010).




Three different reaction pathways were proposed for the reaction mecha-

nism of urea electro-oxidation (Daramola, Singh, and Botte 2010). Among

those three, the reaction pathway expected to have least resistance based

on the rate constants calculated from the transition state theory is:

2 2 2 2 adsCO(NH ) + M [M CO(NH ) ]→ •Reaction 15

2 2 ads 2 ads 2[M CO(NH ) ] + OH [M CO NH NH)] + H O + e− −• → • •Reaction 16

2 ads ads 2[M CO NH NH] + OH [M CO NHNH] + H O + e− −• • → • •Reaction 17

ads ads 2[M CO NHNH] + OH [M CO NHN] + H O + e− −• • → • •Reaction 18

ads 2 ads 2[M CO NHN] + OH [M CO N ] + H O + e− −• • → • •Reaction 19

2 ads ads 2[M CO N ] + OH [M CO OH] + N + e− −• • → • •Reaction 20

ads 2 ads 2[M CO OH] + OH [M CO ] + H O + e− −• • → • Reaction 21

2 ads 2[M CO ] M + CO• →Reaction 22

where M stands for the catalyst – nickel oxyhydroxide (NiOOH) molecule.

Table 9 lists the rate constants for the reaction steps listed above were de-

termined using the transition state theory and the calculated ‘k’ values. Us-

ing the reaction rate constants for these reactions, desorption of carbon

dioxide (CO2) from the NiOOH surface is the limiting (slowest) rate de-

termining step. This can also suggest that after the urea molecule is brokendown on the NiOOH surface, the carbon-based compounds formed in the

process can block the active sites.

Table 9. Kinetic information for the reaction steps involved in urea electrolysis.

Reaction steps Rate constant, k

2 2 2 2 adsCO(NH ) + M [M CO(NH ) ]→ •

6.8 L mol-1 s-1

2 2 ads 2 ads 2[M CO(NH ) ] + OH [M CO NH NH)] + H O + e− −• → • •

1.4 x 1017 L mol-1 s-1

2 ads ads 2[M CO NH NH] + OH [M CO NHNH] + H O + e− −• • → • •

1.1 x 1017 L mol-1 s-1

ads ads 2[M CO NHNH] + OH [M CO NHN] + H O + e− −• • → • •

2.5 x 10-4 L mol-1 s-1

ads 2 ads 2[M CO NHN] + OH [M CO N ] + H O + e− −• • → • •

3.6 x 10-7 L mol-1 s-1

2 ads ads 2[M CO N ] + OH [M CO OH] + N + e− −• • → • •

7.3 x 108 L mol-1 s-1

ads 2 ads 2[M CO OH] + OH [M CO ] + H O + e− −• • → •

1.6 L mol-1 s-1

2 ads 2[M CO ] M + CO• →

4.3 x 10-65 s-1




4.4 Experimental research

The experimental investigation of the reaction mechanism for the electro-

chemical oxidation of urea involved a cyclic voltammetry study using a ro-

tating disk electrode (RDE). The RDE electrode was prepared by electro-

plating Ni over a Ti disk (4 mm diameter and 5 mm thick) using a Watt’s

plating bath. The Ni loading over the Ti disk was 0.5 mg/cm2. Figure 17

shows the cyclic voltammogram for the Ni covered Ti electrode in urea so-

lution (0.33 M urea + 5 M KOH) at various scan rates on the RDE elec-

trode.

The urea oxidation peak current and its potential are plotted against the

scan rate to determine the kinetic information for the electrochemical oxi-

dation of urea. Figure 18 shows a plot of the urea oxidation peak potential vs. a natural logarithm of the scan rate.

Figure 17. Cyclic voltammogram of Ni (0.5 mg/cm2) covered Ti electrode in urea solution. The

urea oxidation peak observed in the forward direction of the scan, whose peak current andoxidation potential varies with the scan rate.

-40

0

40

80

120

160

200

240

280

-0.4 -0.2 0 0.2 0.4 0.6 0.8 1

i ( m

A / c m

2 )

E (V) versus Hg/HgO

10 mv/sec

20 mv/sec

40 mv/sec

60 mv/sec

80 mv/sec

100 mv/sec




Figure 18. Urea oxidation peak potential as a function of scan rate. Cyclic voltammetry using

Ni covered Ti disk in urea solution (0.33 M urea + 5 M KOH).

The transfer coefficient during the oxidation of urea was determined by:

1 p2 p1

2

1E - E = 0.03 log

n

υ

α υ

Eq. 2

where:

Ep1 = peak potential at v1 (V)

Ep2 = peak potential at v2 (V)

v1/v2 = ratio of the potential scan rate

= transfer coefficient

no = number of electrons in the rate determining step (no = 1).

Using the slope from the plot in Figure 18 along with Equation 2, the

transfer coefficient () was calculated to be 0.797. The number of electrons

involved in the urea oxidation reaction can be calculated using the Randle-

Sevcik equation:

( )1 11

5 2 22 p oI = 2.99 x 10 n 1 n AC Dα υ − Eq. 3

where:

Ip = peak current (A)

v = potential scan rate (V/s)

-0.35 -0.30 -0.25 -0.20 -0.15 -0.10 -0.05

-0.010

-0.008

-0.006

-0.004

-0.002

0.000(a)

( E p 2 - E p 1 ) / V

log (υ1/υ2)

Slope = 0.0376




A = electrode area (cm2)

Co = concentration (mol/cm3)

D = diffusion coefficient (cm2/s)

n = number of electrons required for the electrochemical reactionThe diffusion coefficient of the urea molecule in water is 7.57 x 10-11

cm2/s.

Using the transfer coefficient calculated from Equation 2 ( = 0.797) and

the slope of the plot for peak current versus the square root of the scan

rate (Figure 19), the number of electrons (n) required for urea electro-

oxidation was found to be 6.064, which approximates to 6 electrons. This

value for the number of electrons confirms that the urea oxidation reaction

was taking place on the anode electrode as shown in Reaction 4.

4.5 Recommendations for Phase II (reaction mechanisms)

It is recommended that Phase II of this project:

• in molecular modeling, extend a single molecule NiOOH catalyst to the

periodic structure similar to Pt clusters in the ammonia electrolysis

studies

• study the interaction of each intermediate in the reaction steps of the

mechanism with the NiOOH catalyst with periodic structure of NiOOH

• determine the rate constants for all the reaction steps in the mechanism

• experimentally verify the presence of the intermediate and confirm themolecular model using in-situ spectroscopic methods such as Raman,

FT-IR, and XRD spectroscopy.

Figure 19. Peak current for urea oxidation as function of the scan rate. The peak current (Ip)

for urea oxidation is represented versus the square root of the scan rate (v).

0.1 0.2 0.3 0.4 0.50

15

30

45

60

I p

/ m A

υ1/2 /(mV sec-1)1/2

(b)




5 Catalyst Development

5.1 Ni-Rh catalyst

The active electrocatalysts for the oxidation of urea were nickel hydroxide

and nickel oxyhydroxide. The use of nickel electrodes in the urea electroly-

sis experiments led to a drop in the activity of the electrode over a period

of time. The data in Table 9 (p 28) indicate that the loss in electrocatalytic

activity of nickel electrode is attributed to the surface blockage by carbon-

based compounds, where the desorption of carbon dioxide from NiOOH

surface is the rate determining step for the reaction mechanism of urea

electro-oxidation.

This suggests that new and efficient catalysts are required for the electro-

chemical oxidation of urea in alkaline media. One of the catalysts investi-

gated was a rhodium (Rh) deposited nickel electrode, labeled as Ni-Rh,

where Rh has higher affinity to carbon compounds than Ni, and where Rh

is expected to remove the surface blockage on Ni sites caused by carbon

compounds. For comparison, Figure 20 shows the performance of two

electrocatalysts during cyclic voltammetry in urea solution (0.33 M urea +

5 M KOH). The red and purple curves in Figure 20 represent the cyclic

voltammograms of Ni and Ni-Rh electrodes, respectively, in blank solution(5 M KOH). A review of the cyclic voltammograms in urea solution reveals

that the Ni electrode (blue curve) has lower oxidation currents than the Ni-

Rh electrode (green curve). The increase in the urea oxidation current with

the Ni-Rh catalyst over the Ni electrode indicates that the presence of Rh

along with Ni helps to reduce the effect of carbon compounds blocking the

Ni active sites, and to enhance the urea oxidation reaction.

5.2 Large size electrode

The large size Ni-Rh electrodes were prepared using the plating line forthe electrolytic cells assembled to form the 50 W GreenBox. The large size

electrodes were Ni mesh (#40) shaped as an elongated oval (Figure 21).

These electrodes were rinsed thoroughly with deionized water and acetone

before being subjected to Rh plating. The composition of the Rh plating

bath is 1.27 g/L RhCl3 in 1 M NaCl. Rh was electroplated over the Ni mesh

electrodes at a constant cell voltage of 1.566 V. The anode electrodes of the

GreenBox were loaded with 70 mg (0.5 mg/cm2) of Rh, whereas the cath-

ode electrodes were loaded with 140 mg Rh (1.0 mg/cm2).




Figure 20. Comparison of cyclic voltammograms of Ni and Ni-Rh electrodes in urea solution.

The electrolytes used for this comparison are blank solution (5 M KOH) and urea solution

(0.33 M urea + 5 M KOH).

Figure 21. Schematic diagram of the large sized Ni-Rh electrode used in the GreenBox.

The Rh plated Ni electrodes were assembled to form a single cell, which

was tested in blank (5 M KOH) and urea (0.33 M urea + 5 M KOH) solu-

tions under an open cell condition. The testing for the cell consisted of in-

creasing the cell voltage from 1.3 to 1.6 V in staircase fashion and observ-

ing the oxidation current produced. Figure 22 shows the results from

testing a pair of anode/cathode electrodes in blank and urea solution.

5M KOH

5M KOH + 0.33M urea

Rh plated on Ni foil

Plain Ni foil




Figure 22. Potentiostatic experiment of an electrolytic cell in blank and urea solution; the

anode and cathode is Rh deposited Ni mesh electrode.

The maximum current from the single cell in the urea solution is 2.85 A at

1.575 V. For comparison, Table 10 lists the currents from both blank and

urea solutions to illustrate the percentage of actual total current used for

the electrolysis of urea; the remainder is taken for water electrolysis at the-

se high cell voltages.

Table 10. Cell voltage and current for the potentiostatic experiment of single cell in blank and

urea solution.

Cell voltage (V)

Current (A)Current efficiency for urea

electrolysislank solution

(5 M KOH)

Urea solution

(0.33 M + 5 M KOH)

1.3 0.1 0.1 0

1.35 0.08 0.2366 66%

1.4 0.05 0.8507 94%

1.45 0.07 1.4345 95%

1.5 0.1047 1.771 94%

1.55 0.3373 2.4761 86%

1.575 0.6186 2.8539 78%




5.3 Small sized electrode

The development on the deposition of Rh over Ni electrode was studied

using small electrodes of dimensions less than 3 cm x 3 cm. The studiesinvolved optimizing the Rh plating bath and cell voltage for plating only.

Investigation of other parameters were left for Phase II of this project.

The cell voltage required to plate Rh over Ni electrodes was investigated

on a 3 x 3 cm Ni foil. Since a uniform Rh plating is a very important pa-

rameter in the analysis of the catalytic property of this electrode, a plating

apparatus was developed to obtain uniform Rh loading over the Ni foil.

Figure 23 shows the front and the side view of the apparatus, which was

made of polyvinyl chloride (PVC) fittings. The apparatus was submerged

in a 600 mL beaker filled with the Rh plating solution. The Rh plating so-lution flowed through the apparatus, and the streams of solution exited

from the apparatus to converge at the center, creating turbulence. Chang-

ing the flow rate of the Rh solution through the apparatus leads to varia-

tion in the plating voltage.

The counter electrode used in the Rh plating was 5 x 5 cm Pt sheet. The

pretreatment for the Ni foil included sand blasting and a rinse with deion-

ized water and acetone`. After drying the Ni foil in the oven, it was further

subjected to alkaline cleaning (1 M NaOH) and an acid dip (1 M HCl) toremove any oxide layers formed on the Ni foil surface.

Figure 23. Plating apparatus designed for uniform distribution of Rh over Ni electrode,

showing (a) front view of the plating apparatus, and (b) side view of the apparatus (which is

immersed in plating solution) and the electrodes.




The Rh was then plated on Ni electrodes at 5 mA/cm2 for 352 seconds to

apply a plating thickness of 4.6 ± 0.5 mg of Rh. Different cell voltages,

ranging from 1.43 to 1.97 V, for the Rh plating was obtained by varying the

Rh solution flow rate (0.5 to 1.5 L/min) through the plating apparatus. Theplated electrodes were characterized using cyclic voltammetry, X-ray dif-

fraction (XRD) spectroscopy, and (morphologically) using scanning elec-

tron microscope (SEM). Figure 24 shows the SEM images of the Rh plated

Ni electrodes from different plating voltages.

The SEM images of the Rh plating indicate that applying low cell voltages

in the Rh plating process results in full coverage of Rh over the Ni foil and

a cracking of the deposited Rh sheet. However, applying high cell voltage

in the Rh plating process results in the formation of Rh nodules covering

the Ni foil completely. Figure 25 shows the XRD spectra for these Rh plat-ed Ni foils, which indicate the presence of an Rh (111) crystal plane.

The broadness of Rh(111) peak suggests that the (111) planes on the Rh de-

posit are not well defined, and that its ability to catalyze urea electro-

oxidation must be further examined. Preliminary results from the urea

electrolysis using these Rh plated electrodes suggest that a higher intensity

Rh(111) peak in the XRD spectra results in a lower charge for the urea oxi-

dation (Figure 26).

The relation between the Rh(111) peak intensity and the charge for urea

oxidation might indicate that the deposited Rh has more coverage over the

Ni surface. This high Rh coverage would hinder the electrochemical inter-

action between the urea molecule and the Ni sites. Figure 27 shows the re-

lationship between the urea oxidation charge and the Ni(111) peak intensi-

ty, which supports the findings shown in Figure 26, with lower Ni(111)

peak intensity and less urea oxidation charge. In other words, the Ni(111)

peak intensity might have been lowered with more Rh coverage. This will

require further study in the Phase II of this project to confirm these find-

ings.

5.4 Ni-Co hydroxide catalyst

One of the concerns during the electrochemical oxidation of urea is the

overpotential required for this reaction. Theoretically, the oxidation of

urea (Reaction 4) requires -0.46 V vs. SHE, but this reaction will require

0.45 V vs. SHE on pure Ni surface, indicating a large overpotential. The

large overpotential for urea oxidation provides room for a secondary reac-

tion such as the oxygen evolution reaction (OER).




Figure 24. SEM images of Rh plated Ni foil electrodes. The Rh was plated on the Ni foil

electrodes at cell voltages of (a) 1.43 V, (b) 1.51 V, (c) 1.61 V, (d) 1.70 V, and (e) 1.81 V.

Figure 25. XRD spectra of Rh deposited Ni foil. The presence of Rh(111), Rh(200), and

Ni(111) are observed in these spectra.

Figure 26. Urea oxidation charge as a function of Rh(111) peak intensity. The amount of

Rh(111) deposited have influenced the amount of urea molecules oxidized.

Rh (111)Ni (111) Rh (200)

0




Figure 27. Urea oxidation charge as a function of Ni(111) peak intensity. The coverage of Rh

on Ni surface has influenced the amount of urea molecules oxidized.

The use of cobalt (Co) along with the Ni(OH)2 catalyst was investigated to

avoid the OER. (In previous research [Vidotti et al. 2008], nickel-cobalt

hydroxide has been used in urea determination and OER prevention.) Par-

tial substitution of Ni in bulk Ni(OH)2 with Co has shown to improve the

electrochemical property of the catalyst (Pickett and Maloy 1978).

A study was done to determine the composition of the nickel-cobalt hy-

droxide, so that the new catalyst would reduce the overpotential for urea

oxidation. The catalyst (nickel-cobalt bimetallic hydroxide) was co-

deposited on a pretreated Ti substrate. The pretreatment steps for the Ti

substrate involved ultrasonic cleaning with liquid soap solution, 0.01 M

potassium hydroxide, and 0.01 M nitric acid, which was followed by rins-

ing with distilled water and acetone. After cleaning the Ti surface, it was

sandblasted and then again ultrasonically cleaned with distilled water and

acetone. Table 11 lists the composition of the solution used to deposit sixcatalyst samples with different compositions of nickel-cobalt hydroxide.

The composition of the nickel-cobalt bimetallic hydroxide was changed by

varying the concentration of the nitrate salt of the corresponding metal in

the supporting electrolyte (0.1 M KNO3).




Table 11. Composition of deposition solution for nickel-cobalt hydroxide catalyst.

Samples Composition of deposition solution

Sample A 0.1 M KNO3 + 0.01 M Ni(NO3)2

Sample B 0.1 M KNO3 + 0.009 M Ni(NO3)2 + 0.001 M Co(NO3)2

Sample C 0.1 M KNO3 + 0.008 M Ni(NO3)2 + 0.002 M Co(NO3)2

Sample D 0.1 M KNO3 + 0.005 M Ni(NO3)2 + 0.005 M Co(NO3)2

Sample E 0.1 M KNO3 + 0.002 M Ni(NO3)2 + 0.008 M Co(NO3)2

Sample F 0.1 M KNO3 + 0.01 M Co(NO3)2

The surface of the samples was characterized using SEM. Figure 28 shows

the SEM images of the six deposited samples. The addition of Co to the

Ni(OH)2 catalyst resulted in a breakdown of the Ni(OH)2 films on the Ti

substrate. An increase in Co content up to 43% (atomic weight) in thenickel-cobalt hydroxide catalyst resulted in smaller fragmentation of the

catalyst. However, beyond 73% (atomic weight) of Co in the bimetallic hy-

droxide catalyst, large changes were observed; the film structures of the

catalyst became increasing dense.

Figure 28. SEM images of the nickel-cobalt hydroxide electrodes. (a) Sample A, (b) Sample B,

(c) Sample C, (d) Sample D, (e) Sample E, and (f) Sample F.




The chemical composition of the samples was expected to follow the solu-

tion compositions listed in Table 11, but there were appreciable changes in

the composition of the catalyst. Electrochemical characterization of the six

electrodes involves cyclic voltammetry in blank (5 M KOH) and urea (0.33M urea + 5 M KOH) solutions. The onset potential for the electrochemical

oxidation of urea was determined for the six electrodes. Figure 29 shows

the onset potential for urea oxidation as a function of Co content in the

bimetallic hydroxide catalyst. As Figure 29 shows, as the Co content in the

catalyst increased of up to 43% (atomic weight), the onset potential moved

towards lower potential values, which indicates a lower overpotential for

urea oxidation. However, beyond 43% (atomic weight) of Co in the cata-

lyst, the onset potential increased towards that of the pure Ni(OH)2 elec-

trode behavior because Co or Co(OH)2 is not an effective catalyst for the

electro-oxidation of urea.

The addition of 43% (atomic weight) of Co to the pure Ni(OH)2 catalyst

lowered the onset potential for urea oxidation by 150 mV. Constant cell

voltage experiments were performed, between 1.35 and 1.6 V for Sample A

(pure Ni(OH)2) and Sample C (43% Co in Ni(OH)2) with blank and urea

solutions. Catalyst containing Co inhibited the water electrolysis process

and enhanced the urea electrolysis efficiency.

Figure 29. Onset potential for urea oxidation as function of Co content in the bimetallic

catalyst. Sample C with 43% Co in the catalyst has the lowest onset potential.




5.5 Ni(OH)2 nanosheets

The field of nanoscience and nanotechnology has helped in the production

of efficient catalysts for various applications at reduced catalyst quantity

and higher catalytic efficiency. On this basis, the synthesis of two-

dimensional (2D) Ni(OH)2 nanosheets was studied to improve the current

density of the urea electrolysis process and to reduce the onset potential of

the urea oxidation. The synthesis of layered Ni(OH)2 nanosheets required

mixing 5 mmol of sodium dodecyl sulfate (SDS), 2 mmol of nickel nitrate

hexahydrate (Ni(NO3)2 6H2O), and 12 mmol of hexamethylenetetramine

(HMT) with 100 mL of water. This mixture was allowed to react in a sealed

Teflon vessel at 90 °C for 24 hrs.

A green precipitate was obtained after the product solution was centri-fuged at 10,000 rpm for 30 minutes. The layered Ni(OH)2 was washed

thoroughly with distilled water and acetone by shaking and centrifuge.

Figure 30 shows the XRD pattern of the dried layered nickel hydroxide,

which displays a number of Miller indices “00N -type” reflections, suggest-

ing the synthesized Ni(OH)2 were ordered along the layer stacking direc-

tion. Using the reflection angles and Bragg’s law, the interlayer distance

along the c axis was calculated to be 2.67 nm.

Figure 30. XRD pattern of the SDS intercalated layered nickel hydroxide (Wang, Yan, and

Botte 2011).




Adding the layered Ni(OH)2 to a formamide solution and stirring at 200

rpm for 4 days resulted in exfoliated layered Ni(OH)2. The solution with

the suspended exfoliated Ni(OH)2 was centrifuged at 2000 rpm to obtain

nanosheets of the Ni(OH)2 in the supernatant solution. The atomic forcemicroscopy (AFM) was used to study the morphology of the Ni(OH)2

nanosheets. Figure 31 shows the AFM image of the Ni(OH)2 nanosheets,

which has a lateral size ranging from a few hundred nanometers to a mi-

crometer. The thickness of the exfoliated Ni(OH)2 nanosheets was around

1 nm.

The catalytic activity of the Ni(OH)2 nanosheets was analyzed by cyclic

voltammetry. The electrochemical experiments were performed with an

electrode developed by attaching the Ni(OH)2 nanosheets on the glassy

carbon electrode by Teflon binder. For comparison, Figure 32 shows thecyclic voltammograms of the Ni(OH)2 nanosheets in blank (5 M KOH) and

urea (0.33 M urea + 5 M KOH) solution vs. the Ni(OH)2 powder.

Figure 31. AFM image of the exfoliated layered nickel hydroxide nanosheets. The lateral size

of the nanosheets ranged from a few hundred nanometers to a micrometer and the thickness

was around 1 nm (Wang, Yan, and Botte 2011).




Figure 32. Comparison of cyclic voltammograms of the Ni(OH)2 nanosheets modified glassy

carbon electrode and bulk Ni(OH)2 powder modified glassy carbon electrode (Wang, Yan, andBotte 2011).

The peak potential for the urea oxidation was shifted by 100 mV in the

negative direction for the Ni(OH)2 nanosheet modified electrode on com-

parison with the bulk Ni(OH)2 powders. The oxidation current for the

Ni(OH)2 nanosheets modified electrode was 154 mA cm-2 mg-1, which was

170 times larger than the bulk Ni(OH)2 powder. The Ni(OH)2 nanosheets

modified glassy carbon electrode was a better performing catalyst as it

lowered the urea oxidation potential by at least 300 mV from the noble

metal catalysts (Boggs, King, and Botte 2009; Simka, Piotrowski, and

Nawrat 2007; Simka et al. 2009).

5.6 Recommendations for Phase II (catalyst development)

It is recommended that Phase II of this study:

• establish Rh plating conditions on Ni electrodes to obtain predomi-

nantly Rh(111), which would enhance urea electro-oxidation

• scale up the new Rh plating process for preferred crystal plane on the

Ni electrode to the large extended oval shaped electrodes for use in theGreenBox

• scale up of the new catalysts such as nickel-cobalt bimetallic hydroxide

to the large sized electrodes to be used in the GreenBox

• transfer the Ni(OH)2 nanosheets to the large sized electrodes for use in

the GreenBox

• use the Ni(OH)2 nanosheets as a catalyst support material for other

noble metal-based catalyst for ammonia electrolysis, which would ap-

ply to large sized electrodes for the GreenBox application.




6 Application of Ammonia and Urea

Electrolysis in Wastewater RemediationThis project was undertaken to investigate the application of electrolysis of

ammonia and urea as a hydrogen fuel source in support the silent camp

project. The process developed in this work has great potential to provide

an efficient fuel source for PEM fuel cells. However, the electrolysis of

ammonia and urea also has broad applications in military and civilian ap-

plications related to wastewater remediation.

As the global population rises (to an expected 9 billion by 2050), the de-

mand for potable water will increase, while access to fresh water will de-

crease. At the same time, wastewater quantities will exceed the capacity of

existing wastewater management facilities, requiring facility expansions

and quality improvements in expelled water.

Wastewater treatment plants (WWTPs) are net users of energy. In the

United States WWTPs consume an average estimated 21 billion kWh/yr,

which represents 3% of the electric load in the United States. Energy costs

can account for 30% of the total operation and maintenance (O&M) costs

of WWTPs. Continual increases in energy costs in the United States affect wastewater treatment plants just as they do other facilities. Furthermore,

as populations grow and environmental requirements become more strin-

gent, demand for electricity at such plants is expected to grow by approxi-

mately 20% over the next 15 years. Thus, energy conservation is an issue of

increasing importance to WWTPs.

Secondary treatment processes (for the removal of organic matter and ni-

trogen) in WWTPs are responsible for about 60% of the energy consump-

tion in WWTPs. The removal of ammonia (through nitrification/denitrify-

cation technology) consumes an average of 1073 kWh per million gallon

(MG) of water. This value is significant considering that cities with popula-

tions over 2.5 million people commonly require WWTPs with capacities

that exceed 400 million gallon per day (MGD). Currently, wastewater

treatment solutions for ammonia (biological and chemical treatments)

consume a significant amount of energy (3.2 kWh per kg of ammonia re-

moved); have high operational costs ($4 per lb of ammonia removed); re-

quire significant capital investment ($658,000 per MGD), are not flexible

to tighter emissions regulations; and are large in size. (Size alone makes




such systems less feasible in urban areas where there is limited room for

expansion.)

Both traditional and the ammonia/urea remediation systems require addi-tional support systems for their operational execution (SBW 2002). The

GreenBox system offers significant advantages over those systems:

• The GreenBox system is compact in size, and is expected to provide su-

perior efficiencies due to reduction of pumping length runs, and power

for these systems.

• The GreenBox minimizes associated maintenance and upkeep with re-

duced numbers of storage vessels and smaller floor space require-

ments.

• The experimental GreenBox process offers significant energy efficien-

cies over conventional ammonia remediation technologies. The data in

Table 1 (p 5) indicate that the GreenBox process requires between 9

and 17 Wh of energy to recover 1 g of hydrogen gas, depending on pH

and concentration of ammonia. This process destroys 5.67-grams of

ammonia (i.e., from Reaction 3, 2[17/6]), and therefore consumes only

1.59 to 3.00 Wh for each gram of ammonia destroyed. This process is

clearly competitive with the conventional remediation technology,

which requires 3.2 to 5.0 Wh to remove 1 g of ammonia (SBW 2002).

• A further advantage of the GreenBox process is that, in addition to de-

stroying ammonia, it liberates hydrogen gas, which can then be used asfuel in a PEM fuel cell.

• Typically, commercial PEM fuel cells can produce ~14.0 Wh of useful

electricity for each gram of input hydrogen. Since the GreenBox pro-

cess requires between 9 and 17 Wh to produce a gram of hydrogen, the

energy produced by the PEM/GreenBox configuration may represent a

net gain of electrical power. (Note that this does not account for poten-

tial parasitic losses in the GreenBox process.)

Thus GreenBox technology can efficiently destroy ammonia while return-

ing in kind the initial electro-decomposition energy requirements and in

doing so, become a potential net producer of energy. This remediation of

ammonia through the electrolysis process offers several significant ad-

vantages over all other ammonia wastewater remediation technologies (air

stripping, SHARON,* DEMON®,†

* SHARON (Single reactor system for High activity Ammonium Removal Over Nitrite, see:

etc). No other wastewater technology

http://www.dep.state.pa.us/dep/deputate/watermgt/wsm/wsm_tao/InnovTech/ProjReviews/SharonH

iRate.htm

† DEMON (DE-amMONiafication Process), see http://grontmij.com/demon




presently has the capability to produce an emission free (hydrogen) energy

source. The availability of a pure hydrogen fuel source allows the practical

application of PEM fuel cells and ultra high efficient underwater hydrogen

combustion.

Ammonia and urea electrolysis are platform technologies that have signifi-

cant relevance to maintain water resources. Successful development of the

proposed technology will yield an efficient system for onsite treatment that

is simple to operate while providing the added benefit of producing fuels

for electrical and thermal energy generation. The ammonia and urea elec-

trolysis technologies will benefit the DoD by reducing the costs, logistical

burden, and risks associated with wastewater management at FOBs and

other expeditionary outposts.




7 Conclusions and Recommendations

7.1 Conclusions

The electrolysis of ammonia and urea has shown the potential to generate

a high value fuel from a wastewater source. Phase I of this project focused

on determining the kinetics and operating parameters required for the

scale up of ammonia and urea electrolyzers, specifically, by improving the

electrochemical reaction rates for the oxidation of ammonia and urea.

Progress was made by focusing on three important areas: improvement on

the fundamental knowledge of the mechanisms of reactions, catalyst de-

velopment, and construction and scale up of a bench scale electrolyzer.

7.1.1 Improvement on the fundamental knowledge of the mechanisms of

reactions

Experiment has shown that the ammonia electrolysis process consumes less

energy to produce pure hydrogen than urea and water electrolysis. A com-

mercial water electrolyzer requires 49.75 Wh per gram of hydrogen. On the

other hand, an ammonia electrolytic cell with Pt-Rh catalyst on carbon fiber

consumed only 8.6 Wh per gram of hydrogen at 50 °C and 5 M NH3. Even

the highest energy consuming value for the ammonia electrolytic cell (Pt-Ir

catalyst) was 17.3 Wh per gram of H2 at 25 °C and 1 M NH3. Energy savings

using ammonia electrolysis is at least 65.1% more efficient when compared

to that of water electrolysis. In the case of urea electrolysis, an electrolyzer

with Rh plated Ni mesh electrode consumed 34.96 Wh to produce 1 g of hy-

drogen, which is still 29.7% more energy efficient than water electrolysis.

This work performed fundamental studies using molecular modeling

methods and basic electrochemistry techniques coupled with analytical

methods to evaluate the reaction mechanisms for the electrochemical oxi-

dation of ammonia and urea. In molecular modeling, there are two estab-lished reaction mechanisms for ammonia electro-oxidation (Oswin and

Salomon, and Gerischer and Mauerer). The rate constants for the reaction

steps postulated in the Oswin and Salomon mechanism were calculated

using transition state theory. A Pt catalyst model was developed from a

single molecule to a periodic structure (Pt20 cluster) with physical proper-

ties similar to the bulk Pt surface. This Pt surface would be extensively

used in future calculations to confirm the reaction mechanism for ammo-

nia electrolysis.




This work proposed a reaction mechanism for urea electrolysis using the

molecular modeling calculations with a single molecule of NiOOH catalyst.

The rate determining step predicted from transition state theory calcula-

tions is the desorption carbon dioxide from the NiOOH surface, suggestingthe carbon-based compounds could be blocking the Ni active sites from

urea oxidation. It is anticipated that, in Phase II of this project, the struc-

ture of the active catalyst for urea oxidation (NiOOH) would be modeled to

better include the bulk Ni surface details and so to provide a better esti-

mate of reaction rate constant values, which are in closer agreement with

the experimental data.

This work designed an electrolytic cell for ammonia and urea electrolysis

with large rectangular shaped planar electrodes, and then changed the

electrode design to an extended oval shape to allow easier removal of gasesand to avoid the gas build up leading to high back pressure. The electrodes

were assembled to form a single electrolytic cell and the urea electrolysis

was performed to verify the catalytic activity of the electrode (Ni-Rh). The

optimum conditions for hydrogen generation with the single cell system in

0.33 M urea + 5 M KOH solution was to operate the cell at 1.6 V with the

anode electrolyte flow rate of 7.8 mL/min and the cathode electrolyte flow

rate was maintained at 120 mL/min. The optimum conditions for the sin-

gle cell electrolyzer to remove urea from the electrolyte were: (1) to main-

tain the cell voltage at 1.6 V, and (2) to maintain an electrolyte flow rate of

1.1 mL/min for the anode, and 120 mL/min for the cathode.

7.1.2 Catalyst development

The active catalyst for urea electro-oxidation is the Ni(OH)2 and NiOOH

redox couple. During the electrolysis, the observed catalyst activity was re-

duced and researchers predicted surface blockage by carbon compounds.

The presence of Rh along with Ni has enhanced the urea oxidation current.

Platinum group metals have higher affinity towards carbon compounds, so

Rh in the Ni-Rh catalyst helps to preferentially remove the carbon com-pounds from Ni active sites thereby enhancing the urea oxidation current.

Phase I of this project developed two new catalyst materials. Nickel-cobalt

bimetallic hydroxide was synthesized electrochemically. The addition of

43% (atomic weight) of Co to the Ni(OH)2 will lower the onset potential for

urea oxidation. The reduction in onset potential for urea indicates the cell

voltage for the electrode containing nickel-cobalt hydroxide will be less

than Ni(OH)2 electrodes.




Recent progress in the catalyst development was the formation of Ni(OH)2

nanosheets. The Ni(OH)2 is a layered compound that can be exfoliated to

form nanosheets. These Ni(OH)2 nanosheets have a thickness of approxi-

mately 1 nm and a lateral size that varies between a few hundred nanome-ters and a micrometer. A Ni(OH)2 nanosheets modified glassy carbon elec-

trode displayed urea oxidation current 170 times larger than Ni(OH)2

powder modified glassy carbon electrode. The surface area on the Ni(OH)2

nanosheets catalyst is very large. It is anticipated that Phase II of this pro-

ject will determine the capability of the Ni(OH)2 nanosheets as catalyst

and as a support material for other catalysts.

Furthermore, the new developments in the synthesis of nickel nanosheets

can be coupled with the ammonia and urea electrolysis technology. This

work concludes that the nickel nanosheets will support the electrocatalystsused in the ammonia electrolyzer, and in turn has the potential to reduce

the energy consumption during the electrolysis of ammonia to less than

8.6 Wh/g of hydrogen, thus providing higher net energy when combined

with a hydrogen fuel cell.

7.1.3 Construction and scale up of a bench scale electrolyzer

A highlight of the Phase I research work was the construction and testing

of a 50 W electrolyzer known as the GreenBox. This electrolyzer reflects

the linear scalability of the ammonia and urea electrolysis systems as wellas the advancement in the catalyst material development. Assembly of 10

single electrolytic cells into a stack forms a 50 W GreenBox system. The

operation and testing of the GreenBox requires constant monitoring of

such parameters as anode electrolyte flow rate, cathode electrolyte flow

rate, temperature, pH of the electrolyte, cell voltage, current, pressure, and

electrolyte composition. Consequently, a test stand was custom built to

meet the requirements of operating, monitoring, and digital data acquisi-

tion of the GreenBox during electrolysis. It is anticipated that Phase II of

this project will include a more detailed experimental analysis of the oper-ation of the 50 W GreenBox.




7.2 Recommendations for Phase II or future work

In the following areas, it is recommended that Phase II of this project:

•

Electrolysis of ammonia and ureao Use nickel contacts in the preparation of electrodes for the future

experiments.

o Use the nanostructured catalyst such as Ni(OH)2 nanosheets as cat-

alyst and possible support material in the future electrode genera-

tions.

• Test of 50 W GreenBox

o In future experiments, consider other parameters related to the

urea electrolysis in the GreenBox system. (Determination of the ef-

fect of the following variables is currently planned: temperature,

current, flow rates, electrolyte concentration, and urea concentra-tion.)

• Reaction mechanisms

o Calculate the reaction rates for the elementary steps in the Oswin

and Salomon and the Gerischer and Mauerer mechanisms using the

Pt20 cluster modeled.

o Build a NiOOH cluster structure that is more representative of a

larger nickel electrode to provide a better understanding of the oxi-

dation of urea.

o Verify experimentally the presence of the intermediates for both

ammonia and urea electrolysis, and validate the molecular models

using in-situ spectroscopic methods such as Raman, FT-IR, and

XRD spectroscopy.

• Catalyst development

o Establish Rh plating conditions on Ni electrodes to obtain predom-

inantly Rh(111), which would enhance urea electro-oxidation.

o Scale up the new Rh plating process for preferred crystal plane on

the Ni electrode to the large extended oval shaped electrodes for use

in the GreenBox.

o Scale up of the new catalysts such as nickel-cobalt bimetallic hy-droxide, to the large sized electrodes to be used in the GreenBox.

o Transfer the Ni(OH)2 nanosheets to the large sized electrodes for

use in the GreenBox.

o Use the Ni(OH)2 nanosheets as catalyst support material for other

noble metal based catalyst for ammonia electrolysis, which would

transfer to large sized electrodes for the GreenBox application.




Acronyms and Abbreviations

Term Definition

AEC ammonia electrolytic cell

AFM atomic force microscopy

ANSI American National Standards Institute

CERL Construction Engineering Research Laboratory

DFT Density Functional Theory

ERDC Engineer Research and Development Center

FC flow control (valve)

FT-IR Fourier transform infrared (spectroscopy)

HMT hexamethylenetetramine

HQUSACE Headquarters, U.S. Army Corps of Engineers

LFM liquid flow meter)

MG million gallons

MGD million gal/day

NAE National Academy of Engineering

NRC National Research Council [Canada]

NSN National Supply Number

OER oxygen evolution reaction

OMB Office of Management and Budget

PEM Proton Exchange Membrane

PVC polyvinyl chloride

RDE rotating disk electrode

RDS rate determining step

SAR same as report

SDS sodium dodecyl sulfate

SEM scanning electron microscope

TR Technical Report

UCEAO The University Clean Energy Alliance of Ohio

URL Universal Resource Locator

US United States

XRD X-ray diffraction




References and Publications

References

Alexandrova, A. N., and W. L. Jorgensen. 2007. Why urea eliminates ammonia ratherthan hydrolyzes in aqueous solution. Journal of Physical Chemistry B 111(4):720-730.

Barrios, A. M., and S. J. Lippard. 2000. Interaction of urea with a hydroxide-bridgeddinuclear nickel center: An alternative model for the mechanism of urease.Journal of the American Chemical Society 122(38):9172-9177.

Becke, A. D. 1993. A new mixing of Hartree–Fock and local density ‐functional theories.

Journal of Chemical Physics. 98:1372. doi:10.1063/1.464304.Benini, S., W. R. Rypniewski, K. S. Wilson, S. Miletti, S. Ciurli and S. Mangani. 1999. A

new proposal for urease mechanism based on the crystal structures of the nativeand inhibited enzyme from Bacillus pasteurii: Why urea hydrolysis costs twonickels. Structure 7(2):205-216.

Boggs, B. K., and G. G. Botte. 2009. On-board hydrogen storage and production: Anapplication of ammonia electrolysis. Journal of Power Sources. 192(2):573-581.

Boggs, B. K., R. L. King, and G. G. Botte. 2009. Urea electrolysis: Direct hydrogenproduction from urine. Chemical Communications, pp 4859-4861.

Cooper, M., and G. G. Botte. 2006. Hydrogen production from the electro-oxidation of

ammonia catalyzed by platinum and rhodium on raney nickel substrate. Journalof the Electrochemical Society, 153(10):A1894-A1901.

Daramola, D. A., D. Singh, and G. G. Botte. 2010. Dissociation rates of urea in thepresence of NiOOH catalyst: A DFT analysis. Journal of Physical Chemistry A.114(43):11513-11521.

de Vooys, A. C. A., M. T. M. Koper, R. A. van Santen, and J. A. R. van Veen. 2001. The roleof adsorbates in the electrochemical oxidation of ammonia on noble and trans-ition metal electrodes. Journal of Electroanalytical Chemistry. 506(2):127-137.

Estiu G., and K. M. Merz. 2007. Competitive hydrolytic and elimination mechanisms inthe urease catalyzed decomposition of urea. Journal of Physical Chemistry B.111(34):10263-10274, http://pubs.acs.org/doi/abs/10.1021/jp072323o

Estiu, G., and K. M. Merz. 2004a. Enzymatic catalysis of urea decomposition: Eliminationor hydrolysis? Journal of the American Chemical Society. 126(38):11832-11842.doi:10.1021/ja047934y,http://pubs.acs.org/servlet/linkout?suffix=ref3/cit3&dbid=20&doi=10.1021%2Fjp105159t&k

ey=10.1021%2Fja047934y

Estiu, G., and K. M. Merz. 2004b. The hydrolysis of urea and the proficiency of urease.Journal of the American Chemical Society. 126(22):6932-6300.doi:10.1021/ja049327g,http://pubs.acs.org/servlet/linkout?suffix=ref4/cit4&dbid=20&doi=10.1021%2Fjp105159t&k

ey=10.1021%2Fja049327g




Estiu, G., D. Suarez, and K. M. Merz. 2006. Quantum mechanical and moleculardynamics simulations of ureases and Zn β-lactamases. Journal of ComputationalChemistry. 27(12):1240-1262.

Fearon, W. R. 1926. The biochemistry of urea. Physiological Reviews. 6(3):399-439.

Gaussian Inc. 2003. Gaussian 03 (software), http://www.gaussian.com/

Gerischer, H., and A. Mauerer. 1970. Untersuchungen zur anodischen oxidation vonammoniak an platin-elektroden. Journal of Electroanalytical Chemistry.25(3):421-433.

Hay, P. J., and W. R. Wadt. 1985. Ab initio effective core potentials for molecularcalculations. Potentials for the transition metal atoms Sc to Hg. Journal of Chemical Physics. 82(1):270. doi:10.1063/1.448799.

Hay, P. J., and W. R. Wadt. 1985b. Ab initio effective core potentials for molecularcalculations. Potentials for K to Au including the outermost core orbitals. Journalof Chemical Physics. 82(1):299. doi:10.1063/1.448975.

Hehre, W. J., L. Radom, P. V. R. Schlayer, and J. A. Pople. 1986. Ab Initio MolecularOrbital Theory. New York: John Wiley & Sons.

M. B. Biradar. 2007. Design, scale-up, and intergration of an ammonia electrolytic cell with a proton exchange membrane (PEM) fuel cell. Chemical and BiomolecularEngineering. Athens,OH: Ohio University.

Marquez, Andres I., Frederic Vitse, and Gerardine G. Botte. 2004. Theoreticalinvestigation of the electro-oxidation of ammonia. Electrode Processes VII, 206th Meeting of The Electrochemical Society, p 203,http://electrochem.org/dl/ma/206/pdfs/2000.pdf

Musiani, F., E. Arnofi, R. Casadio, and S. Ciurli. 2001. Structure-based computationalstudy of the catalytic and inhibition mechanisms of urease. Journal of BiologicalInorganic Chemistry. 6(3):300-314.

National Research Council (NRC) and National Academy of Engineering (NAE). 2004.The hydrogen economy: Opportunities, costs, barriers, and R&D needs.

Washington, DC: The National Academies Press.

Oswin, H. G., and M. Salomon. 1963. The anodic oxidation of ammonia at platinum black electrodes in aqueous KOH electrolyte. Canadian Journal of Chemistry.41(7):1686-1694.

Pickett, D. F., and J. T. Maloy, 1978. Microelectrode studies of electrochemically coprecipitated cobalt hydroxide in nickel hydroxide electrodes. Journal of the

Electrochemical Society. 125(7):1026-1032,http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=JESOAN0001250000

07001026000001&idtype=cvips&gifs=yes&ref=no

Santen, R. A., and J. W. Niemantsverdriet. 1995. Chemical kinetics and catalysis. New York: Plenum Press.

SBW Consulting, Inc. 2002. Energy benchmarking secondary wastewater treatment andultraviolet disinfection processes at various municipal wastewater treatmentfacilities. San Francisco, CA: Pacific Gas and Electric Company,http://www.cee1.org/ind/mot-sys/ww/pge2.pdf




Simka, W., J. Piotrowski, and G. Nawrat. 2007. Influence of anode material onelectrochemical decomposition of urea. Electrochimica Acta. 52(18):5696-5703.

Vidotti, M., M. R. Silva, R. P. Salvador, S. I. C. de Torresi, and L. H. Dall'Antonia. 2008.Electrocatalytic oxidation of urea by nanostructured nickel/cobalt hydroxide

electrodes. Electrochimica Acta. 53(11):4030-4034.doi:10.1016/j.electacta.2007.11.029,http://www.sciencedirect.com/science/article/pii/S0013468607013941

Simka, W., J. Piotrowski, A. Robak, and G. Nawrat. 2009. Electrochemical treatment of aqueous solutions containing urea. Journal of Applied Electrochemistry.39(7):1137-1143.

Wadt, W. R., and P. J. Hay. 1985a. Ab initio effective core potentials for molecularcalculations. Potentials for main group elements Na to Bi. Journal of ChemicalPhysics. 82:284. doi:10.1063/1.448800.

Wang, Dan, Wei Yan, and Gerardine G. Botte. 2011. Exfoliated nickel hydroxide

nanosheets for urea electrolysis. Electrochemistry Communications. 13(10):1135-1138. doi:10.1016/j.elecom.2011.07.016.

Publications

The following sections lists technology transfer activities (posters, confer-

ence lectures, and journal publications) derived from this research.

Fourth Annual UCEAO (The University Clean Energy Alliance of Ohio)

conference at Columbus, Ohio (April 20-21, 2010)

Hydrogen combustion applied to domestic water heaters (Poster)

Improvement of AgO cathode for Primary batteries (Poster)

Electro-oxidation of urea using rotating disk electrode (RDE) (Poster)

Theoretical study of the kinetics of ammonia oxidation on Pt(111) anodes (Poster)

Cathode optimization for hydrogen production through ammonia electrolysis (Poster)

217th The Electrochemical Society Meeting at Vancouver, Canada (April 25-

30, 2010)

Theoretical investigation of ammonia oxidation kinetics on Pt(111) (Poster)

218th The Electrochemical Society Meeting at Las Vegas, Nevada (October

10-15, 2010)

Electric field effects on the adsorption of NH3 on Platinum for electrochemical oxidation(Poster)

Electrochemical remediation of swine wastewater (Poster)

Identifying and adapting a solid polymer electrolyte for ammonia electrolysis (Talk)

Electro-oxidation of urea on NiOOH electrode (Talk)




In-situ Raman spectroscopic study of ammonia electro-oxidation in alkaline media (Talk)

Ammonia oxidation on platinum anodes: a DFT study of two mechanisms (Talk)

1st Advisory board meeting of the Center for Electrochemical Engineering

Research (CEER), Ohio University at Athens, Ohio (November 11, 2010)

Electric field effects on NH3 adsorption on Pt(111) electrodes (Poster)

In-situ Raman spectroscopic study of ammonia electro-oxidation in alkaline media(Poster)

Economic feasibility for residential power production using ammonia electrolytic cell(AEC) (Poster)

Evaluation of catalyst support materials for ammonia electrolysis (Poster)

Electrochemical remediation of swine wastewater (Poster)

Electro-oxidation of urea on NiOOH electrode (Poster)

Preparation and characterization of Ni/Co(OH)2 electrodes for the electro-oxidation of urea in alkaline solution (Poster)

Chemical conversion of urea to ammonia (Poster)

219th The Electrochemical Society Meeting at Montreal, Canada (May 1-6,

2011)

Evaluation of catalyst support materials for ammonia electrolysis (Talk)

Electrodeposition of rhodium on nickel electrodes for urea oxidation (Talk)

Denitrification of wastewater through ammonia electrolysis (Talk)

Journal publicationsg

Daramola, Damilola A., Deepika Singh, and Gerardine G. Botte. 2010. Dissociation ratesof urea in the presence of NiOOH catalyst: A DFT analysis. Journal of PhysicalChemistry A. 114(43):11513-11521.

King, Rebecca L., and Gerardine G. Botte. 2011. Hydrogen production via ureaelectrolysis using a gel electrolyte. Journal of Power Sources. 196(5): 2773-2778.

King, Rebecca L., and Gerardine G. Botte. 2011. Investigation of multi-metal catalysts forstable hydrogen production via urea electrolysis. Journal of Power Sources.196(22):9578-9584.

Wang, Dan, Wei Yan, and Gerardine G. Botte. 2011. Exfoliated nickel hydroxidenanosheets for urea electrolysis. Electrochemistry Communications. 13(10):1136-1138.



R E P OR T DOC U ME N TATION P AG EForm Approved

OMB No. 0704-0188Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining tdata needed, and completing and reviewing this collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducinthis burden to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 222024302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currevalid OMB control number. PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS.

1. REPORT DATE (DD-MM-YYYY)

26-01-2012

2. REPORT TYPE

Final

3. DATES COVERED (From - To)

4. TITLE AND SUBTITLE

Electro Decomposition of Ammonia into Hydrogen for Fuel Cell Use:Phase-I Study

5a. CONTRACT NUMBER

5b. GRANT NUMBER

5c. PROGRAM ELEMENT

6. AUTHOR(S)

Gerardine G. Botte and Carl A. Feickert

5d. PROJECT NUMBER

MIPR

5e. TASK NUMBER

5f. WORK UNIT NUMBER W917PM10612813

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

U.S. Army Engineer Research and Development Center (ERDC)Construction Engineering Research Laboratory (CERL)PO Box 9005,Champaign, IL 61826-9005

8. PERFORMING ORGANIZATION REPORNUMBER

ERDC/CERL TR-12-1

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSOR/MONITOR’S ACRONYM(S)

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HQDA, Deputy Assistant Secretary of the Army for Research and Technology 2800 Crystal Drive, 5th Floor, Suite 513 Arlington, VA 22202

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Approved for public release; d is tri bution is unl imi ted.

13. SUPPLEMENTARY NOTESR eport Documentation Pa ge (SF 298)

14. ABSTRACT

This work was undertaken to create an efficient process for electrolyzing ammonia, by clarifying the electrolytic decomposition path

ways of ammonia and urea. This project demonstrated the feasibility of using ammonia and urea electrolysis technologies to produchydrogen as a potential fuel source for the fuel Proton Exchange Membrane (PEM) fuel cell back up power for training facilities and

soldier camps, under the “Silent Camp” initiative. This was achieved with scaling of bench scale electrolyzer to a 50 W electrolyzer

system known as the “GreenBox.” The construction of the 50 W GreenBox depended on the development of the catalyst and funda-

mental understanding of the reaction mechanisms for ammonia and urea electrolysis. Significant progress in catalyst development wachieved by using chemical and electrochemical preparation techniques, and using the various state-of-the-art analytical methods fu

ed through this project. A new synthesized material–nickel hydroxide nanosheets–has shown potential to be catalyst for urea electro

sis and catalyst support for ammonia electrolysis. The energy consumption for the ammonia electrolysis using the nickel based nanostructured electrodes is anticipated to be lower than 8.6 Wh per gram of hydrogen gas produced. The low energy consumption wprovide a significant advantage when the GreenBox is combined with fuel cells.

Documents

Electro Decomposition of Ammonia Into Hydrogen